Optional checks

Add the ability to have optional checks that are not run by default. This will allow us to have checks with high false positive rates, etc., that most people probably don't want to see but others do.

Instead of
```ruby
Checks.add MyCoolCheck
```
use
```ruby
Checks.add_optional MyCoolCheck
```

An optional check can be run via `-t MyCoolCheck` or `-A` to run all checks.