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`.

thefuck/rules/yarn_help.py

@@ -0,0 +1,13 @@
+import re
+from thefuck.utils import for_app
+
+
+@for_app('yarn', at_least=2)
+def match(command):
+    return command.script_parts[1] == 'help' and ('for documentation about this command.' in command.stdout)
+
+
+def get_new_command(command):
+    fix = re.findall(r'Visit ([^ ]*) for documentation about this command.', command.stdout)[0]
+
+    return 'open ' + fix