Deprecate the RedCloth API compatibility layer

Hello,

This is just a pull request that deprecates the RedCloth API compatibility layer. We can move the `RedcarpetCompat` class to its own file and remove its documentation from the README file. The only drawback is that people relying on the `RedcarpetCompat` class should now put `require 'redcarpet/compat'` in their code but this is no-sense to use it over `Markdown`.

We can't throw deprecation warnings as this would mess the users' output so let's simply remove any reference to it for now. We will be able to remove it once we reach Redcarpet 4.0.

Have a nice day!