Feat/add command cli

[tdejager]

Let's try that again..

Command management interface from the CLI.

[ruben-arts]

[rarts@fedora test]$ pixi command add test
✔ Added command test: 

[rarts@fedora test]$ pixi c alias test
@ Added alias test: 

I don't think these should do something.

[baszalmstra]

Fix #101