Add typescript type declarations

[mDibyo]

Adding typescript declarations to make this easier to use with typescript projects. :)

[mDibyo]

@tappleby Could you take a look at this and lmk what you think?

[tappleby]

It appears to make sense, but dont know enough about typescript. Will have a look and see.

[mDibyo]

@tappleby Ok thanks. Fyi, its based on a similar PR I made on another repo and subsequent changes:

• PR: Add typescript type declarations tshelburne/redux-batched-actions#11
• Subsequent commits (check changes made in last 4 commits): https://github.com/tshelburne/redux-batched-actions/commits/master/package.json

[mDibyo]

Btw, thanks for creating redux-batched-subscribe. I am finding it super useful!