Add an optional predicate filter to `get_segment` and `get_segments`

[JocelynDelalande]

This is a helper to ease the kind of parsing (semi-manual searching) that pydifact allows.

[nerdoc]

This is a very nice trick, b< using lambda callables instead of regexing etc. Really well done. I'll merge this PR, however, It should be done some documentation how to use it properly, just an lambda example like in the tests.