Add HTML type

[mattlenz]

This pull request adds support for the text/html type.

[gaearon]

Please test this change on the examples in React DnD repo.
You can run

npm install
npm start

and then open http://localhost:8080 and go through the examples.

[gaearon]

This breaks the examples using URLs because HTML type wins.
I don't think we can take it without significantly changing how native types are handled.

We need to either start treating them as the same type, figure out a way to support many times in a nice way, or make native type configuration injectable so apps can choose what they care about. I'm going to close this PR because it doesn't address this bigger issue, but feel free to maintain a fork or propose an API for handling these cases.