Add new external command: unpack

`brew-unpack` is inspired by the `unpack` command of Haskell's Cabal.
`brew unpack <formulae ...>` will fetch the source code archives of one or more
formula and extract them into subfolders of the current working directory. An
alternate root directory may be specified by invoking the `--destdir` flag.

This provides some nice advantages for inspecting source code compared to
`brew install -i -f <formula>`:

  - The extracted source code won't be deleted when the Ruby process
    terminates---which allows for long-term experimentation.

  - The user has control over which directory the code is extracted into.

  - No install process is initiated, so accidentally typing the equivalent of
    `exit 0` will not cause `brew` to mess with an existing installation.

  - Several formulae may be unpacked at once.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

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