Add option to create missing rule docs

[bmish]

--init-rule-docs to create any missing rule docs.

Fail if there are no rule docs to create so this option isn't left in place indefinitely.

This would only be useful for someone who hasn't created any docs for their rules yet. In that scenario, this would help save time in getting started with documentation.

The reason we need an explicit option for this (and we don't want to automatically create missing rule docs by default) is because the rule docs might live at a non-standard path and we just didn't detect them. If the plugin has a non-standard rule doc path, the user just needs to specify --path-rule-doc.

Follow-up to #78.