Support issue-links from within markdown files

[staabm]

Describe the feature
not sure this is within the scope of this tool, feel free to close.

I think it could be usefull when text in markdown like #123 which renders to real links in github comments/pr description, would also be supported when looking at changelog.md files.

see e.g. https://github.com/sabre-io/xml/blob/master/CHANGELOG.md

it might also be usefull to link the @nickname notations within these files.

whether this only work for files named changelog.md CHANGELOG.md or similar - or whether this could also make sense in other .md files could be discussed

[stefanbuck]

Although I like the idea, I'm just slightly concern that such a feature could easily cause side effects. Maybe it's worth creating a quick prototype or add it behind a flag so people can try it out and provide feedback.