Typescript support

[magnattic]

Is your feature request related to a problem? Please describe.
Right now this project cannot be used with Typescript properly.

Describe the solution you'd like
It should provide support for typescript:

• accept typescript database files (db.ts)
• provide types for it's own module (json-graphql-server)

Describe alternatives you've considered
The only alternative I see is to provide typings myself and use the programmatic variant with ts-node to get TS files for the database to work.

[fzaninotto]

Nice. Feel free to open a PR adding TS support!