Optimize _lp_in_multiplexer

Use the return code of the function to return the value instead of
stdout.
Major fix for brackets in terminal multiplexer

Optimize terminal multiplexer detection by moving it to the initial
loading instead of at each prompt display: the terminal multiplexer is
always a parent process and that can not change during the shell life.

This also fixes a major leak: LP_MARK_BRACKET_OPEN was growing at each
prompt display when running in a terminal multiplexer.
(check with: echo "$LP_MARK_BRACKET_OP" | od -c )
Merge branch 'fix/multiplexer-brackets' of https://github.com/dolmen/liquidprompt into develop

Conflicts:
	liquidprompt
Update the doc about colored brackets