#195 - support for metadata value matchers

[SebastianMC]

• implementation completed, tested locally on a dedicated vault
• need to create unit tests for the new and modified areas before release

Sample syntax from test vault:

---
sorting-spec: |
  with-metadata: biblio status matching: value(unread)
  with-metadata: biblio status matching: value(reading)
  with-metadata: biblio status matching: value(complete)
  def
---

---
sorting-spec: |
  separator top
  with-metadata: n n matching: range(,123)
  separator 123
  with-metadata: n n matching: range[123,456]
  separator 456
  with-metadata: n n matching: range(456,)
  separator end
---