Add new `only_4xx` key to report just 4xx errors

An ask from GitHub. A few changes here:

* Dropping the `as_link_array` option. Proofer should be smart enough to know that the first arg is an array.
* The option setup was convoluted, and is now cleaner 
* A new `only_4xx` option is possible, which, if set, ensures that Proofer only reports errors for 4xx level problems.

/cc @penibelst @aroben