"lint" command

[erunion]

We should have a lint command, that does everything that generate does when compiling your documentation. Right now, validation is halted at the first error, so instead of that, this command should build up a list of all the errors in your documentation and spit out a failure or success at the end.

Having this, it'll allow people to integrate this into their build processes alongside things like PHPUnit, PHPCS, or static analysis checkers.