Add package options for plugins

[Alexnortung]

It would be nice if I could change which version of a plugin to use (pinning or using unstable/stable) while using nixvim.

I would like to be able to do this in the same manner as in most NixOS modules in nixpkgs handles this, by adding a package option.

[pta2002]

Good idea, though it'd definitely be a relatively big undertaking. I can get started on it though! Also ensure that all new plugins have that option

[Alexnortung]

I can also do some work on this and will open some pull requests ;)