Allow disabling custom rules in code

[steverichey]

This doesn't seem to work in 0.8.0.

[jpsim]

At the moment, you'll have to disable all custom rules:

// swiftlint:disable custom_rules

But this is something we should fix so that you can disable custom rules individually. We should also adapt the rules command to include custom rules in the table and allow swiftlint rules my_custom_rule to work.