Fix memory issue with plugin interface

We should make a copy of the signature, as we don't want
the caller to handle the lifespan and the costs should be
reasonable in all cases. Changing experimental API is fun.

Author: mgreter

src/sass_functions.cpp

@@ -26,6 +26,7 @@ extern "C" {
 
   void ADDCALL sass_delete_function(Sass_Function_Entry entry)
   {
+    free(entry->signature);
     free(entry);
   }
 

src/sass_functions.hpp

@@ -7,7 +7,7 @@
 
 // Struct to hold custom function callback
 struct Sass_Function {
-  const char*      signature;
+  char*            signature;
   Sass_Function_Fn function;
   void*            cookie;
 };