Changelog: add issue and commit links
Changelog: add issue and commit links

For future reference, the regex replacements used:

Issues:
#([0-9]{3,4})
[$0](https://github.com/HaxeFlixel/flixel/issues/$1)

Commits:
\(([a-z0-9]{7,8})\)
([$1](https://github.com/HaxeFlixel/flixel/commit/$1))

[skip ci]

HaxeFlixel/flixel