Make tinyformat aware of locales.

Without all this, you will get situations where format("%g",123.45)
might return "123,45" if it happens to be running on a machine with
locale "fr_FR", for example. If that is written to an output file that
is later read on a computer using a locale with '.' as the decimal mark,
it could mis-parse as "123.0", leading to hilarious and tragic results.

It seems like the safe thing is to force the string-returning varieties
of tinyformat to use classic locale ('.'  decimal), with an optional
means for the odd users to purposely specify a different (or native)
locale. It also seems that the best policy for the stream-appending
varieties of format() is to use the local already associated with those
streams (C++ lets each stream have its own locale).

1. format() to an existing stream (including cout) is now documented to
   honor the stream's existing locale (as it always did).

2. format() that returns a string now will always format the string using
   the classic "C" locale, in particular meaning that floating point
   numbers will use '.' as the decimal mark, regardless of the "native"
   locale set by the OS or process that may have an unexpected choice of
   decimal point.

3. Add a new variety of format() that returns a string, but which takes
   an explicit locale, for cases where you don't want the "C" locale
   (for example, when generating strings that will appear in a UI that
   you want localized properly). You can make this variety use the
   current locale by passing std::locale() as the first argument (the
   default constructor of std::locale just grabs the global process
   locale).

Author: lgritz

tinyformat.h

@@ -943,6 +943,7 @@ TINYFORMAT_FOREACH_ARGNUM(TINYFORMAT_MAKE_MAKEFORMATLIST)
 #endif
 
 /// Format list of arguments to the stream according to the given format string.
+/// This honors the stream's existing locale conventions.
 ///
 /// The name vformat() is chosen for the semantic similarity to vprintf(): the
 /// list of format arguments is held in a single function argument.
@@ -955,23 +956,40 @@ inline void vformat(std::ostream& out, const char* fmt, FormatListRef list)
 #ifdef TINYFORMAT_USE_VARIADIC_TEMPLATES
 
 /// Format list of arguments to the stream according to given format string.
+/// This honors the stream's existing locale conventions.
 template<typename... Args>
 void format(std::ostream& out, const char* fmt, const Args&... args)
 {
     vformat(out, fmt, makeFormatList(args...));
 }
 
 /// Format list of arguments according to the given format string and return
-/// the result as a string.
+/// the result as a string, using classic "C" locale conventions (e.g.,
+/// using '.' as decimal mark).
 template<typename... Args>
 std::string format(const char* fmt, const Args&... args)
 {
     std::ostringstream oss;
+    oss.imbue (std::locale::classic());  // force "C" locale with '.' decimal
+    format(oss, fmt, args...);
+    return oss.str();
+}
+
+/// Format list of arguments according to the given format string and return
+/// the result as a string, using an explicit locale. Passing loc as a
+/// default-constructed std::locale will result in adhering to the current
+/// "native" locale set by the OS.
+template<typename... Args>
+std::string format(const std::locale& loc, const char* fmt, const Args&... args)
+{
+    std::ostringstream oss;
+    oss.imbue (loc);
     format(oss, fmt, args...);
     return oss.str();
 }
 
 /// Format list of arguments to std::cout, according to the given format string
+/// This honors std::out's existing locale conventions.
 template<typename... Args>
 void printf(const char* fmt, const Args&... args)
 {
@@ -1023,6 +1041,16 @@ template<TINYFORMAT_ARGTYPES(n)>                                          \
 std::string format(const char* fmt, TINYFORMAT_VARARGS(n))                \
 {                                                                         \
     std::ostringstream oss;                                               \
+    oss.imbue (std::locale::classic());                                   \
+    format(oss, fmt, TINYFORMAT_PASSARGS(n));                             \
+    return oss.str();                                                     \
+}                                                                         \
+                                                                          \
+template<TINYFORMAT_ARGTYPES(n)>                                          \
+std::string format(const std::locale& loc, const char* fmt, TINYFORMAT_VARARGS(n)) \
+{                                                                         \
+    std::ostringstream oss;                                               \
+    oss.imbue (loc);                                                      \
     format(oss, fmt, TINYFORMAT_PASSARGS(n));                             \
     return oss.str();                                                     \
 }                                                                         \