Create index.ts when generating files from a directory

[galah92]

I'm using the CLI as follows:

json2ts -i './*.json' --output -o ../contracts

Where in my current directory there are multiple JSON Schema files. In the contracts directory I'm getting multiple d.ts files. I'd like to be able to reference them together, and to autogenerate an index.ts file while will re-export everything inside the directory.
If there's an easier way to achieve that I would be happy to hear about it.

Thank you

[bcherny]

Dup of #258