[Feature suggestion] [Homebrew] Add an option to replace nano with micro

[JamesYeoman]

Description of the problem or steps to reproduce

Vim on Homebrew has the option to make vi a symlink to vim. This isn't currently a thing for micro but it would be nice to have that functionality.

Specifications

An install option is the MVP for this suggestion. It makes sense to start off with because there may be people who still want to use nano while using micro. However, if you feel that it would make more sense to have it as the default, I'm not going to argue

Commit hash: 1856891
OS: Mac OSX 10.12.6
Terminal: iTerm2