Add visual feedback other than "Control" lighter in mode line

[mrkkrp]

The package would benefit from good visual feedback similar to that used in Evil mode. That is, when we're in “normal” mode let cursor be in form of box (by default, it will be configurable), while in “insert” mode let it be in form of thin bar.

I made heavy use of this package in short time and it would be absolutely impossible to use without this visual feedback. If you agree that this will be an improvement, I will include necessary changes into pull request I'm working on.