Add metadata to force a plugin to load immediately after another

[Ortham]

Discussed in loot/skyrim#190 and loot/skyrim#245, the idea is to have something like

- name: B.esp
  directly_after: A.esp

which has the effect that LOOT will sort B.esp after A.esp with no other plugins between them.

Open Questions

• Should directly_after be invalid for regex entries? It only makes sense for one match to use the value, but the regex entry may be for a set of exclusive plugins anyway.
• How would this interact with the plugin aliasing feature proposed in #46?
• How to handle merging metadata entries that specify different directly_after values?
• How to handle the case where plugin masters and 'hard' metadata like after and req entries form a cycle with directly_after?
• How to implement this - should it be part of the graph sort, or done as a post-topological sort "override" step?

@Kaenguru-Knobel-Kette has noted that it would be good idea to get the opinions of other compatibility patch authors on this, to see if their expectations or use cases differ.