TypeScript type definitions

[DamianoMagrini]

Added an index.d.ts file that contains type definitions (as requested by @sammkj in #28).

I have to admit that I didn't really check that everything is correct. Still, this is just a port of @types/react-helmet v4, so, as long as this is a port of react-helmet v4, they should be correct.

Maybe the file should be put inside src, and the scripts should be changed to copy the file at build time? I've seen that many repos have it in the root directory though.

^{(this is my first pull request I sincerely hope I haven't messed anything up)}

Edit: closes #32.

[DamianoMagrini]

I've just noticed that this is incompatible with Preact X. To make it work, JSX should be replaced by preact.JSX.
By the way, why are all tests failing? From the logs it looks like a certain /etc/init.d/xvfb is missing.

[Download]

I think there is probably some small issue with the tests, because they worked when I last looked at this.

Please become a maintainer, then you can merge it yourself :) Join the discussion at #34