Save IFS at startup to restore it in _lp_set_prompt

We already restore IFS to its default value. This patch only changes the
way it is done to avoid a call to 'echo' in a sub-shell at runtime.
Fixes #267.
Merge branch 'develop' of github.com:nojhan/liquidprompt into develop

* 'develop' of github.com:nojhan/liquidprompt:
  Save IFS at startup to restore it in _lp_set_prompt

Conflicts:
	liquidprompt