Fix external tmux command without display

[MacGyverNL]

I spent entirely too long today fixing my own instance of #1560, because I'm not running alot in X11 and #460 and 2a7d1fd make alot simply fall back to non-spawning behaviour silently.

Initially I special-cased tmux outside of X11 in alot/commands/globals.py, but then I found out that I'm supposed to write a hook. So I did that, but I'd like to document it publicly.

There's also a bug where the thread result of that hook is not respected because self.in_thread gets set directly.

• Ensure the thread result of the touch_external_command hook is not overridden.
• Provide an example touch_external_command hook and corresponding shell script for tmux outside of X11. (Thanks to @cjpbirkbeck in Edit mail in a different terminal tabs than the main alot windows #1560 (comment))
• Log a warning if spawn is requested outside of X11 but no hook is set, rather than fall back silently to non-threaded mode.

[pazz]

Looks good to me. thanks!