annotation filters with cli arguments

[eWert-Online]

This implements the suggestion from @tsnobip and @mjambon in #324.

• The command line can specify multiple tags (-tag producer -tag res)
• The atd annotations can specify multiple tags (<ocaml tag="producer" tag="foo" name="Foo">)
• An annotation that doesn't specify a tag is assumed to carry all the tags
  (<ocaml name="Foo"> is selected by -tag bar but <ocaml tag="foo" name="Foo"> is not selected by -tag bar)

The tags are currently implemented by removing all attributes from the AST which do not match the cli argument(s).
This is the easiest way I could think of to implement that feature. If there is a better way or this leads to problems, please let me know 🙂

I am not sure, if the schema changes I made, make sense. I just allowed <... tag="foo"> fields in every attribute location (except for doc attributes).

PR checklist

• New code has tests to catch future regressions
• Documentation is up-to-date
• CHANGES.md is up-to-date