Allow to specify CodeMirror's syntax mode as MIME

This is required for e.g. Scala, where the mode is given as
text/x-scala, but the actual implementation is in clike mode.
This wouldn't be an issue, but IPython loads modes lazily, so
you need both mode name and MIME to resolve correct file and
configure CodeMirror.
Merge pull request #6808 from mattpap/codemirror_mime

Allow to specify CodeMirror's syntax mode as MIME