Add typescript definitions

[sangdth]

Add typescript definitions file index.d.ts, also there were some changes to eslint config to make it work with typescript keywords.

[sangdth]

I realized that the file tsconfig.json is useless. I will delete it in next commit. Just wondering, is there anything else I need to do as well? Also I could pump up the version, couldn't I?

Thanks.

[doowb]

Thanks! I'll review this shortly.

You don't need to bump the version, that's better done after the code is merged in.

[jaruszewskiG]

Bump, waiting for this PR to be merged.