Yarn likes to keep its documentation online, rather than in `yarn help`
output. For example, `yarn help clean` doesn't tell you anything about
the `clean` subcommand. Instead, it points you towards
https://yarnpkg.com/en/docs/cli/clean
This rule detects when that happens, and suggests opening the URL. One
caveat is the currently only OSX is supported, as Linux uses `xdg-open`
instead of `open`.
