On large mercurial projects, using `hg status` to show dirty prompt
causes significant delay.

This commit checks a local hg config value of `oh-my-zsh.hide-dirty` to
skip dirty check.

Users who wish to skip dirty check can add this to their `.hg/hgrc`
file.

```
[oh-my-zsh]
hide-dirty = 1
```

This config value uses the same naming as ones found for git, in file
lib/git.zsh.