don't process VCS function if root

they do not appear in the result anyway, and they can be quite slow
LP_ENABLE_VCS_ROOT to enable use of VCS features with root
LP_PS1_PREFIX used to add arbitrary prompt before anything else
added a CUSTOMIZING THE PROMPT section
Merge branch 'ps1-prefix' of https://github.com/dr4Ke/liquidprompt into develop
Default prefix put a classical prompt as title

Update the doc and themes accordingly
Optimize _lp_battery: no sed
Merge pull request #87 from dolmen/fix/batt

Optimize _lp_battery: no sed
git: fix variable leaks
_lp_git_branch: faster (less external programs calls)
Merge pull request #88 from dolmen/fix/git

More efficient git parsing (2)
Add an option to hide the logged user

LP_USER_ALWAYS=1 will display the user, even if he is the same than the logged one.
Defaults to 1 (always display the user), set to 0 if you want to hide the logged user (it will always display different users).