Add GitHub issue link filter

[Ghabry]

Thats a bit rough for now, but it works ;). Please give feedback.

Converts stuff like player#1234 to [#1234](https://github.com/EasyRPG/Player/pulls/1234 "Player: Save game and move route fixes")

Workflow:

1. Create .cache, then execute make_cache.rb (TODO: Add to Makefile, move to a better subdirectory)
2. make

TODO:

• Only generate the cache once. How to trigger regenerations?
• Unknown issue numbers will crash
• Will always emit markdown (is there a way to figure out the context (html or md) in filter?
• Code duplication
• Maybe merge liblcf and Player datasource and use an additional key?