Add tmux support

[jchilders]

This adds a --tmux option allowing for git-radar to be used in a tmux status prompt.

[wkrsz]

Works for me.

I had to tweak status command, otherwise git-radar would always run in the same directory, regardless of pane:

set -g status-right '#(cd "#{pane_current_path}" && git-radar --tmux)'

Also also had an issue where the lightning character added also created a newline breaking the status line. This did not happen with any other emoji:

As a workaround I disabled "use unicode version 9+ widths" setting in iTerm.