ENH: support elif: with selectors

[h-vetinari]

Sometimes we need to distinguish things in more than two ways, the most common case being linux-osx-win. It should be possible to write

        files:
          - if: linux
            then:
              - something_linux
            elif: osx               # <--
            then:
              - something_osx
            else:
              - something_win

rather than having to slice things into repeated if-conditions.

[wolfv]

Same problem as with match that you could only have a single "elif" as duplicate keys are forbidden. I would also like to see a real life use case because i didn't really have a need when writing recipes.

[h-vetinari]

you could only have a single "elif" as duplicate keys are forbidden.

Ugh, right. Even a single elif would be better than none though (IMO)

I would also like to see a real life use case because i didn't really have a need when writing recipes.

Happens all the time to me that I need to distinguish 3 or 4 out of (linux,osx,unix,win). Of course it's possible to work around this by opening a new - if:, but that's extra cognitive overhead - correctly parsing multiple if-statements to check which ones do or don't apply is easy to get wrong (which was the main issue I wanted to solve with #1155)

[wolfv]

I think we need to come up with something better than a single elif :)