Add hooks stub types

[jleclanche]

Touches #591

They're far from complete but better than nothing.
I had a lot of trouble with the code as there's a bunch of type issues throughout it. I would highly recommend moving the project to typescript, it'd be cleaner and easier… :)

[coveralls]

Coverage remained the same at 74.448% when pulling 478020e on jleclanche:master into 503d9e1 on i18next:master.

[coveralls]

Coverage remained the same at 74.448% when pulling 478020e on jleclanche:master into 503d9e1 on i18next:master.

[jamuhl]

thank you for the initial version for hook typings

[mondayCoding]

Not sure if relevant but I tweaked index path locally in node_modules to find typings.
Thank you for these, I was waiting for types to get to testing.

3 | import {Namespace, ReactI18NextOptions, TransProps} from "./index";

=>

3 | import {Namespace, ReactI18NextOptions, TransProps} from "./src/index";

[stunaz]

@jleclanche how are we suppose to use the hooks in typescript?
when I do const [t, i18n] = useTranslation();

later i want to get the current language with i18n.language

I get this typing error:

Type error: Property 'language' does not exist on type '{} | on type '{} | i18n'.
  Property 'language' does not exist on type '{}'.  TS2339{}'.  TS2339