Feature - add typescript declarations

[dpnolte]

Hi,

Thanks for this awesome library.

With this PR, I'd like to add typings to the package.

There were a couple of issues with typing withESI:

• Return type - With declarations enabled, typescript generated an error: the esi property cannot be private/protected in the HOC class. That is, exported anonymous classes can't have private or protected members if declaration emit is enabled, because there's no way to represent that in a .d.ts file. I resolved it by providing an return type to withESI React.ComponentClass<P & IWithESIProps>
• propTypes - The problem with this return type is that the static propTypes member did not account for child properties as provided by the generic variable P. This generated another typescript error. I resolved it by converting the propTypes member first to unknown and then to the proper WeakValidationMap type to account for HOC props and the children props.

[dunglas]

Awesome! Thanks @dpnolte