The added library parses output of the hg (mercurial) command, and outputs
the same variables (which should be customized by theme) in your prompt.
I added an example of these variables to michelebologna theme.
This patch should uniform all the behaviour of all themes regarding mercurial.
In fact, some themes implement their own hg parsing *inside* the theme.
This should be better implemented in a library, as happens with git library.
