fixed stacktrace line printing to reset color at EOL and prevent 'color bleed' to next line.

Perhaps this was by design(?) but I found it distracting.

Author: timmaffett

lib/src/printers/pretty_printer.dart

@@ -270,7 +270,7 @@ class PrettyPrinter extends LogPrinter {
 
     if (stacktrace != null) {
       for (var line in stacktrace.split('\n')) {
-        buffer.add('$color$verticalLine $line');
+        buffer.add(color('$verticalLine $line'));
       }
       buffer.add(color(_middleBorder));
     }