CodeMirror allow all config to be passed to JS from Ruby

Pass all of the config keys to the JS widget to allow you to pass ANY codeMirror config that you would like not limited to theme and mode.

https://github.com/sferik/rails_admin/blob/master/lib/rails_admin/config/fields/types/code_mirror.rb#L12 exposes a config option that can be set on a per app level but ANY config option that is set is ignored.

This patch fixes that.