Fix various spelling / grammar / consistency errors
Fix one more grammar error
README: fix missing guard around liquidprompt for bashrc instructions

Fixes https://github.com/nojhan/liquidprompt/issues/291
liquidpromptrc-dist: fix wrong quotes
implementing issue #110

as the activation of GIT_PS1_SHOWUPSTREAM option does for __git_ps1
the _lp_git_branch_color now compute how many commit we are ahead or
behind of our remote reference.