Adds support to process custom fields

[apachler]

This PR adds support to process custom fields by following ruleset

Ruleset for Custom Field:
  match: True
  metadata_regex: 'Eingegangen (?P<entry_day>\d{1,2}).(?P<entry_month>\d{1,2}).(?P<entry_year>\d{4})'
  metadata_postprocessing:
    title: "Test Custom Fields Functionality"
    custom_fields:
      Eingegangen: '{{entry_year}}-{{entry_month}}-{{entry_day}}'

This ruleset will add a custom field named Eingegangen with content matched by metadata_regex.