darkvlue.vim: reformat

I couldn't stand looking at all those horribly aligned tabs.  Now everything
is in a set order with no tabs.
darkblue.vim: add cterm support for Pmenu
midnight.vim: add SignColumn
vimrc: enable gitgutter

I find the sign column annoying, and sadly it's not possible to do line
highlighting without the sign column.  Therefore, I have disabled gitgutter by
default but added a simple \gg mapping to toggle it on/off.