`lorri pin` command

[target]

`lorri pin` command

[Profpatsch]

@WhoMeNope I think https://github.com/nmattia/niv does all of this (and more) better than what we could ever implement in lorri.

We should add a link to it in a “befriended projects” section of the readme.

[Profpatsch]

This is a cool idea for a drive-by contribution. If somebody wants to take it, we can discuss a design.

[tinybeachthor]

I'm missing this functionality sorely.
I was thinking about the design a bit and maybe this could be just a flag for the init comand (lorri init --pin), which would just output shell.nix with the pinning boilerplate included, either pointing to the latest commit, or maybe leaving the commit empty for the user to edit.

@Profpatsch, what do you think?

[tinybeachthor]

@Profpatsch Niv is looking great, thanks!
Adding it to the readme so it's easier to find from here would be very nice.