Add utility function _lp_bool

_lp_bool converts a return code (default to $?) into a "true"/"false"
value.
Fix #304: _lp_jobcount_color() when is tmux not installed

Refactor _lp_jobcount_color() to avoid running either 'screen' or 'tmux'
if they are not installed.

Conflicts:
	liquidprompt
More _lp_jobcount_color refactoring

Conflicts:
	liquidprompt
Simplify git working tree detection
Remove variable LP_LIQUIDPROMPT, redundant with LP_OLD_PS1

Conflicts:
	liquidprompt
Fix _lp_bool
_lp_shorten_path: clean variable usage inside (( ))
Quoting fixes
bash: save and set shell option promptvars
Fix bracket marks on linux tty