171 metadata value extractors

[SebastianMC]

#171 support for extractors of metadata values to sort by (transformed) part of metadata value

A partially working version, not to be merged until completing the below:

Done (when opening this PR):

• defined new syntax to allow for specification of metadata value extractor
  • !!! DOESN'T allow for spaces in metadata names
• extended the sorting spec parser to detect and recognize the new syntax
• defined two extractors (and their syntax) to support dd/mm/yyyy and mm/dd/yyyy date formats extraction
• extended the data structures generated by the parser to carry the parsing result in a form ready to use by the sorting engine
• extend the sorting engine to use the metadata extractor if it was specified

Remains to be done:

• see how to extend the new syntax to allow spaces in metadata names
  • rather a blocker issue, if not fixed: so far spaces in metadata names were allowed
• add error handling in parser for the new syntax
• make the code production quality:
  • improve readability (e.g. now there are ...name1, ...name2, ...extractor1, ...extractor2, etc.)
  • refactor the structures, newly introduced, maybe the pre-existing which became overloaded
  • better naming
  • DRY
  • minimize the risk of regression bugs
• create the required new unit tests for extended parser and sorting engine features