Add "no operation" plugin

[alexkiro]

We have a few plugins that we don't need, so it would be good to have a NOOP plugin implemented for these. For example if the user uses the ifplugin statement this should not fail for them.

Current list:

• Plugin::MIMEHeader - this is include in the default ruleset
• Plugin::Check - SA requires this plugin to be loaded for anything to actually work, we don't require it