Add morph filter-Functions without conditionals

[TobiasNx]

Morph has:

contains, not-contains, equals, not-equals, blacklist, whitelist
that can operate directly on a certain value.

Fix only has filter for that. Otherwise you have to create an if-conditional to remove fields that are not matching.

e.g.

  if all_match("language", "Mehrsprachig")
    remove_field("language")
  end

This is too complicated in my opinion.

It should be:
not-equals("language", "Mehrsprachig")

We should adopt the morph filter functions.

[fsteeg]

See also boolean and control flow operators in Catmandu.

[TobiasNx]

By now I think the filter mechanisms can be done sufficiently by conditionals + remove_field. Only blacklist and whitelist could be conditionals that would be interesting, but also could be constructed via lookup with delete option and exists conditional.