Brewfile

There's a [large demand](https://www.google.com/search?btnG=1&pws=0&q=%22brewfile%22+%7C%7C+%22brewdle%22#pws=0&q=%22brewfile%22+%7C%7C+%22brewdler%22) for Brewfile integration, an [incredibly popular use-case](https://github.com/mathiasbynens/dotfiles/blob/master/.brew), and there are tons of people in the Ruby niche using [Brewdler](https://rubygems.org/gems/brewdler). It's even been asked for in our GitHub issues (#16664 & #5250), so I thought I'd make a pull request.

~~It's written so that `brew install` (like `npm install` or `bundle`) without any arguments searches for and uses a Brewfile (optional case), which would look like:~~

It's written so that `brew bundle` executes each line of a `Brewfile` prepended with `brew `, so that you would tap and install like so: 

```
# This Brewfile taps and installs gif2mp4
tap christianbundy/tap
install gif2mp4
```


~~It also uses the Brewfile for `brew uninstall` without arguments, so that Homebrew dependencies can be shared quickly and easily with projects.~~ I'm open to suggestions on implementation, I just found it useful today and thought the community might as well.