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.
liquidprompt: code style fix on the whole file

Fix all "if [[ ... ]] ; then" to "]]; then"
Update CHANGES
Merge branch 'feature/git-commits-behind' into develop

Git: show the number of commits behind.
Similar to GIT_PS1_SHOWUPSTREAM=verbose.
Implementation by François Schmidts (@jaesivsm).
https://github.com/nojhan/liquidprompt/pull/269