Remove z plugin

Never ended up using this.
Remove Mercurial plugin

Haven't used `hg` in years.
Add DSL for Homebrew formulae and casks

In order to extend Dot to support installing and managing GUI applications,
add a DSL `formula` and `cask` to specify that a formula or cask should
be installed with Homebrew.

This may be Mac-specific, but it is hopefully implemented in such a way
that similar DSLs could be implemented for other OS vendors (should I
ever decide I want to support both).
Add installation setup for Homebrew

In the event you're installing Dot on a brand new Mac, teach Dot to be
smart enough to install a C compiler if one is not available and then
installing `brew` and the `brew casks` extension.
Homebrew-ize all the things

Add setup scripts for installing various applications via the Homebrew
DSL. This will hopefully make it a lot faster to get set up on a new
computer.