ignore ruby version files
show nonzero exit statuses
another attempt at muting tmux powerlines
enable copy and paste to work with tmux
Clear vim backgrounds

http://snk.tuxfamily.org/log/vim-256color-bce.html
do not show mode indicator

Avoid duplication since Vim-powerline already handles this information.
set the correct color scheme for powerline
enable toggling between light and dark themes

The light theme is more useful for projectors while the dark is more useful for monitors and laptops.