OverwriteHandler flag needs to be at individual handler

[stevehu]

The current generator only respects OverwriteHandler flag as All or Nothing. We need to change the generator to check if an old handler exists before applying the logic if overwrite it or not. In this case, we can allow new endpoints to be added to the specification and get them generated even OverwriteHandler flag is false.

[stevehu]

Relate to #164

[stevehu]

The logic has been changed for overwriteHander, overwriteHanderTest and overwriteModel flag from config.json file.

If the flag is false and the file exists, then skip generating the file. With this change, you should set these flags to false most of the time and only set them to true if you want to overwrite existing files.