bugfix: avoid recurisve prompt command
bugfix: avoid recurisve prompt command

The bug was occuring when sourcing the liquidprompt a second time while it has
already been set. A set variable is now set to avoid double sourcing.

dan-cleinmark/liquidprompt,tsbarnes/liquidprompt,jaesivsm/liquidprompt,enrdstrm/liquidprompt,joshuarubin/dotfiles,timhyson/liquidprompt,pkkolos/liquidprompt,joshuarubin/dotfiles,joshuarubin/dotfiles,alexprengere/liquidprompt,BobHood/liquidprompt,dtschan/liquidprompt,nojhan/liquidprompt,milouse/liquidprompt,lyoshenka/liquidprompt,dpilone/liquidprompt,tkuminecz/liquidprompt,jonathanjsimon/liquidprompt,kenrachynski/liquidprompt,mathieumd/liquidprompt,tstone2077/liquidprompt,colinlieberman/liquidprompt,smadhueagle/liquidprompt,joshuarubin/dotfiles,tocomo/liquidprompt,illwieckz/liquidprompt,laurentbardin/liquidprompt,elementalvoid/liquidprompt