Remove mods by their slugs

[theRookieCoder]

Tasks

• [x] Store the slug when adding a mod
• [x] Fetch the slugs and update the config when listing verbosely to allow older configs to take advantage of this feature
• [x] Implement removal by slug in the remove command

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The Problem

We can currently remove mods by their names or IDs, but not by their slugs.

Your Solution(s)

The slug will have to be additionally stored in the config to keep the process network free.

[theRookieCoder]

Added tests (583a371853da67073b7f7e8713b63e006e3683c7) and documented in the README too (fbfe84aae269a7f269c752deee843997c6e5e67a)!

[ok-coder1]

Please close? #462