Working around a Vim bug that causes flickering.

If the user had a hidden buffer and a recent version of Vim, the screen would
flicker every time the user typed. This was caused by a Vim bug.

On every key press, we end up calling GetUnsavedAndCurrentBufferData(), which
calls GetBufferOption( buffer_object, 'ft' ). If the buffer_object represents a
hidden buffer, Vim would flicker.

This would happen because we'd call "buffer_object.options[ 'ft' ]" in recent
versions of Vim, and that line of code causes Vim to flicker. I don't know why.
We're extracting the 'ft' value without going through buffer_object.options, and
that works just fine.

Fixes #669.