Add function to disable mc undo into undo-list.

When user enables mc mode we need to add function to disable mc mode
into undo-list. When users starts to hit 'undo' at some point mc-mode
will be disabled.
Merge pull request #175 from lexa/master

Add function to disable mc undo into undo-list.
Revert undo-feature for multiple-cursors

re #169