bashrc: Allow pip usage only in a virtualenv
tmux.conf: Convert short format vars to long ones

Short format string variables are deprecated in tmux 1.9
tmux.conf: Remove '-c' from the new-window binding

tmux 1.9 has learned '-c' in the new-session command.
tmux.conf: Replace foo-{fg,bg} with foo-style

From the tmux 1.9 changelog: "The various foo-{fg,bg,attr} commands have been
deprecated and replaced with equivalent foo-style option instead."