Add --now option to esi offer list command

[larsks]

The --now option filters offers to those that have availability start
"right now". This supports the common use case of "I would like to lease a
machine at this moment".

[tzumainn]

I was about to say that it probably makes sense to ensure that we don't specify both --now and --availability-range - but now I wonder whether we should let users simply say --availability-range now?

[larsks]

but now I wonder whether we should let users simply say --availability-range now?

<whinge>But that's so much tyyyyyyyyping.</whinge>

How about we let users specify --availability-range now, but also allow --now as a shortcut? My argument is that this seems like a very common use case, and if we're not going to make it the default behavior we should at least make it as accessible as possible.

[tzumainn]

but now I wonder whether we should let users simply say --availability-range now?

<whinge>But that's so much tyyyyyyyyping.</whinge>

How about we let users specify --availability-range now, but also allow --now as a shortcut? My argument is that this seems like a very common use case, and if we're not going to make it the default behavior we should at least make it as accessible as possible.

Hahahaha - actually that's fair! In that case, no need for --availability-range now; maybe just a check that a user hasn't specified both --availability-range and --now?

[larsks]

I've re-implemented this PR so that it really is shorthand for --availability-range $(date -Iseconds) 9999-12-31T00:00:00. Is that sufficient? There's no warning if you use both --now and --availability-range, because you could just as easily use --availability-range multiple times and we don't issue an error in that case.

[tzumainn]

Fair enough!