Added -f/--formulae FORMULAE to make the mirroring of individual package/resource trees simpler.

It doesn't do dependency resolution, though, which makes it easy to accidentally only mirror the toplevel package + resources. Just grabbing Formula#deps should be sufficient.

1. Download all stable source packages, resources, and external patches, saving them with unique pathnames (Homebrew gives use the SHA256 of everything, so proto://checksum.ext is probably easiest).
   1. Exclude packages that can't be mirrored over HTTP(S). For the time being, this just means SVN.
2. Wrap Homebrew so that:
   1. All urls are rewritten to the local mirror
   2. All download strategies are rewritten to CurlDownloadStrategy or GitDownloadStrategy

There are a few different problems associated with "shallow" git clones (i.e., git clone --depth 1):

1. Dumb HTTP mirrors can't serve them (we circumvent this during mirror fetches by stripping --depth and the trailing depth, see b83e48b)
2. brew fetches them by default for a few domains:

  SHALLOW_CLONE_WHITELIST = [
    %r{git://},
    %r{https://github\.com},
    %r{http://git\.sv\.gnu\.org},
    %r{http://llvm\.org},
  ].freeze

This then causes us to mirror the shallow clone, which then explodes when we try to fetch it.

Candidate solutions:

• Add some extra git magic to prep_location! to unshallow our mirrored repos
• Add/extend the git shim to remove shallow clone functionality during the mirroring process