Add noop linter for demonstration.

The noop linter is just that, a noop, but shows how a linter works and
verifies that they are loaded correctly.  It can probably be removed in
the future but acts as a nice little stub.
Add the invalid_package_name linter.

This linter removes from the passed contents any that have an invalid
package name as their first tuple member.  Simply cleans up what portage
would complain about being invalid anyway (and if it doesn't it's
definitely a bug in this linter).