support 'code' in hasSelectionInBlock (see react-draft-wysiwyg)

[lefnire]

I'm new to Draft, I don't know if code-block is a standard and should be followed, or if you get to make your own or what. https://github.com/jpuri/react-draft-wysiwyg inserts code, so this adds support for that (alternatively ~currentBlock.getType().indexOf('code') in case other editors use codeblock or whatever).

[coveralls]

Coverage remained the same at 99.265% when pulling ebcd057 on lefnire:master into c219dc5 on SamyPesse:master.

[lefnire]

Ah, <pre> maps to code-block, and <code> to code. So this PR should cover everything