brew: add contributed tap commands to PATH.

This means that taps root and `cmd` directories are added to the PATH.

This should enable migration of some of our contributed commands into taps (e.g. `homebrew-boneyard`) and make it easy for third parties to be able to maintain these. It might also make stuff easier for existing tools like e.g. `brew-cask` and `boxen`.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
I would say we either want `cmd` or `Command` (to match `Formula`), but not the root. Then just

```rb
Dir["#{HOMEBREW_LIBRARY}/Taps/*/*/cmd"]
```

so we don't have to worry about this trailing slash business?