Mercurial Prompt Support

Adds new library to provide similar functions to existing git library. Works in the same way, provides `hg_prompt_info` and `hg_prompt_status` that do the same thing as their git counterparts.

Usage:

Set appropriate `ZSH_THEME_HG_PROMPT_*` variables and use `hg_prompt_status` in a theme. Ugly looking example provided in `lib/mjh.theme.zsh` just uses the existing git ones.