Remove Lokaltog/vim-powerline from Bundle list

powerline was interfering with and was superseded by vim-airline.  For
whatever reason vim-powerline turned into a large Python fancy prompt
system.
Make airline powerline font env conditional

Looks for a $POWERLINE_FONT environment variable to be set before
turning on powerline fonts in vim.  Maybe this will allow me to use one
branch between all my environments.