Add dynamic aliases functionality

[rhopfer]

My proposal to add dynamic aliases (see #240). Please check the documentation I wrote because my english is bad. Suggestions are welcome.

[alerque]

Thanks for taking the time to contribute this yet. Overall the concept looks really interesting and I would even suggest that (if we go this route) the existing predefined aliases should be dropped in favor of a default alias config file that would be superseded by a user config if any. I have some concerns that the regex based approach to managing the content of the aliases file might be a bit complex and possibly brittle.

That being said before I review this too deeply or we make a decision I have to ask ... did you consider storing aliases in Git's config system? It's possible to set and retrieve arbitrary keys in there and since a working Git config is already an absolute requirement that might be a way to avoid the overhead of having a separate alias file handling system. Not that we're afraid of more dotfiles of course ;-)

[alerque]

Just for reference, when reviewing this lets compare what GitHub did with their official CLI and its gh alias.

[RichiH]

Also Git's; I suspect everyone will know this, but just to make sure: https://git-scm.com/book/en/v2/Git-Basics-Git-Aliases

[alerque]

Just in case it turns out useful later, it seems this PR's (currently only) commit is a squash of all the bits in this branch.