Json schema (#4369)

* Added empty generator for json schema (idl_gen_json_schema.cpp)
#4360

* JsonSchemaGenerator: output of tables implemented

current problems:
- typenames are not correct
- array types need to be deduced

#4360

* JsonSchemaGenerator: Corrected generation of typenames
Current problems: enum types not written correctly
#4360

* JsonSchemaGenerator: Added generation of enum types
#4360

* idl_gen_json_schema.cpp: Write required properties to schema
#4360

* idl_gen_json_schema.cpp: Export Types including namespace
#4360

* idl_gen_json_schema.cpp: Fixed Json format
#4360

* idl_gen_json_schema.cpp: Formatted according to google code style
#4360

* Checked in monster_test.bfbs with changes from master

* Added idl_gen_json_schema.cpp in CMakeLists.txt

* generate_code.bat: Added generation of json schema

* Added todo.md

* generate_code.sh: Added generation of json schema

* Addressed some review issues
- removed command line arg -S
- removed new lines
- fixed codestyle in template functions
- removed usage of stringstream
- idented json schema
#4360

* removed auto in idl_gen_json_schema.cpp

* idl_gen_json_schema.cpp: changed iterator declarations to auto
#4360

* deleted todo.md

* idl_gen_json_schema.cpp: Removed keyword "override" so that vs2010 can compile

* idl_gen_json_schema.cpp: switch statement in GenType handeles all enum-members

* idl_gen_json_schema.cpp: Removed cerr output

* idl_gen_json_schema.cpp: Avoid vector copying

* idl_gen_json_schema.cpp: Fixed identation of json schema output

* idl_gen_json_schema.cpp: Do not output empty descriptions

include/flatbuffers/idl.h

@@ -380,6 +380,7 @@ struct IDLOptions {
     kJson   = 1 << 7,
     kBinary = 1 << 8,
     kTs     = 1 << 9,
+    kJsonSchema = 1 << 10,
     kMAX
   };
 
@@ -716,6 +717,12 @@ extern bool GeneratePython(const Parser &parser,
                            const std::string &path,
                            const std::string &file_name);
 
+// Generate Json schema file
+// See idl_gen_json_schema.cpp.
+extern bool GenerateJsonSchema(const Parser &parser,
+                           const std::string &path,
+                           const std::string &file_name);
+
 // Generate C# files from the definitions in the Parser object.
 // See idl_gen_csharp.cpp.
 extern bool GenerateCSharp(const Parser &parser,