parse >2.x docker-compose.yml versions (the basics)

[CpuID]

Second attempt at #497 - at a bare minimum to be able to accept files with 2.1, 2.2, etc for now, and stub out where we would put 3.x support later.

This is by no means a comprehensive implementation, but would most likely fix:

#490
#486
#474
#441

And at least error elegantly for #421 for now.

[CpuID]

Note: the credit for most of this goes to @cheapRoc

[CpuID]

This really could use test coverage around some 2.x and 3.x version numbers, I don't have the cycles to touch it today. I'll see how things go soonish.

[vdemeester]

In a near future I think there would be better way to do that (à-la docker/cli), but if it unblocks people, let's get that in.

LGTM 🐯

[CpuID]

awesome, thx for merging so fast :)

[jwreagor]

Thanks @CpuID. Shame my project fell through but glad I could help here in some way.