Add yarn_help rule

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`.

README.md

@@ -250,6 +250,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `workon_doesnt_exists` – fixes `virtualenvwrapper` env name os suggests to create new.
 * `yarn_alias` – fixes aliased `yarn` commands like `yarn ls`;
 * `yarn_command_not_found` – fixes misspelled `yarn` commands;
+* `yarn_help` – makes it easier to open `yarn` documentation;
 
 Enabled by default only on specific platforms: