Add :provider hiccup element

[dawranliou]

Fix for #19

This PR is to:

1. Add react/create-context thin wrapper arround react/createContext
2. Add custom hiccup element :provider to avoid js interop
3. Update the context example to demonstrate the usage

[lilactown]

This is great! I need to think of a good place to document this, but the code looks good

[lilactown]

I'm going to go ahead and merge this and then figure out docs before next release.

[dawranliou]

That sounds good to me. Thanks for merging this. Let me know if there's anything else I can do to improve this feature, like improving the docs.