Add homebrew formula file for easier OSX installation

Hi!

I started using ttygif and noticed the project didn't had a homebrew formula.
As it makes the project installation even easier I added one. It is very basic and does nothing fancy, but it allows user to install ttygif by running:
```bash
$ brew install https://github.com/icholy/ttygif/blob/master/ttygif.rb
```

This PR updates the README accordingly.

Cheers.