[Feature] Define possible options to run a just command arg

[ManiMozaffar]

I think it make sense to have an easy way to define options, for example:

up service:
...

but i would like to have:

up service={"service_a","service_b",...}

which should be also seen in --help command.

[casey]

I think this is a kind of a dupe of #857. But it would be easier to implement, since it's just valid options for an argument, so I'll leave it open.