Add draft for filtering design

This is a proposal how to do filtering of a system description in Machinery. Feedback is appreciated. Please add it as comments in this pull request.

I'm a bit confused about that one. I suppose those filters are stored automatically when a command is run, otherwise the information which filter was used would easily be lost. But the last sentence sounds as if they also were re-applied on the next machinery run automatically. That would be very annoying and confusing, though, in my opinion. Can you please clarify how that's gonna work?

And are those filters that are store in the `meta` section the same that are modified by the user as described in the `Managing filters` section? Because that again would be confusing because users could modify those filters after they ran machinery so that the filter in the description does no longer match the filter that was applied to the description content.

 If those are separate stored filters, where are the user-managed filters stored then?