Added a "strict JSON" mode to the text generator and compiler

This will add quotes around field names, as required by the official
standard. By default it will leave quotes out, as it is more readable,
more compact, and is accepted by almost all JSON parsers.
The -S switch to flatc turns on strict mode.

As per rfc 7159.

Change-Id: Ibabe9c8162c47339d00ec581d18721a2ba40c6d0
Tested: on Windows.

samples/sample_text.cpp

@@ -46,7 +46,8 @@ int main(int /*argc*/, const char * /*argv*/[]) {
   // to ensure it is correct, we now generate text back from the binary,
   // and compare the two:
   std::string jsongen;
-  GenerateText(parser, parser.builder_.GetBufferPointer(), 2, &jsongen);
+  GenerateText(parser, parser.builder_.GetBufferPointer(),
+               flatbuffers::GeneratorOptions(), &jsongen);
 
   if (jsongen != jsonfile) {
     printf("%s----------------\n%s", jsongen.c_str(), jsonfile.c_str());