Allow specifying a list of checks that should run

This reverses the default assumption of "everything enabled". A common request from people getting started is the ability to run only certain checks (e.g. I want to start by just fixing the `file_mode` issues) rather than getting overwhelmed by the firehose.  This is technically possible by manually disabling all the other checks but it's not a nice process. This patch allows the user to specify just the check(s) they want to enable.

Find all the classes/defines that aren't documented
```
puppet-lint --only-check documentation /etc/puppet/modules
```