Showmode option

This is the same feature as #7.
vim-airline or powerline users almost always have their showmode info to false, as the mode is shown inside the statusline anyway. Of course, they don't have support to show submodes, so it makes sense to have an option to show the submode by force.

I have it implemented a little differently than @vheon - more in line with the general style of your plugin.

Closes #7