initial commit for brew-bundle-dir

The idea behind `brew bundle-dir` is that it will run through a directory (either defined via `export BUNDLE_DIR` or `~/.brews.d`) looking for `*.brewfile` files. Similar to the way boxen manifests are typically organized, one would symlink, git submodule or subtree repositories under `~/.brews.d` where one or more `*.brewfile` files can be found.

This should allows for personal and project-specific brewfiles.

These brewfiles get run via `brew bundle`.

Looking for further discussion and potential idea for this.

ATM, this is working well for me.

After looking at boxen and noticing the complexity therein, I realized that most of that already comes OOTB with homebrew, just need a way to allow people to easily organize shared manifests.