Add project_name top-level config key

[pipe01]

Resolves #745

I suppose the JSON schema change should go on a different version but I'm unsure about how to do that.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" [email protected]:pipe01/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[terjebraten-certua]

Duplicate of #3118 and #5369

[ulyssessouza]

As commented in #6815 (comment), this is an already refused feature.

[artnaseef]

Why is this feature "refused"?

[pipe01]

No clue, it's extremely useful