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](https://github.com/wilmoore/dotfiles/tree/master/active/.brews.d) and project-specific brewfiles.

These brewfiles get run via `brew bundle`.

Looking for further discussion and potential ideas 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 and drive shared manifests.

## options

- `--count` provides a count of formula found in all brewfiles found in PREFIX.
- `--help` (although, for some reason, `brew` does not pass this along to the script)
- `--list` lists `*.brewfiles` under `PREFIX`
- `--edit` opens `*.brewfiles` found under `PREFIX` using whatever is set in `$EDITOR` environment variable.