Use TERM 'xterm-256color' on non-tmux shells

'screen-256color' TERM can be problematic in some terminal emulators
(e.g. ubuntu/terminator) when the shell is out of tmux session.

This commit, following after 60c40932, makes the current shell
be detected detect whether it is inside a tmux session, and TERM
set to 'screen-256color' or 'xterm-256color' accordingly.