++ should only aggregate compatible projects

[jroper]

This PR depends on #3 being merged first.

When a user does:

++2.11.5 compile

Currently we build all projects with 2.11.5, regardless of whether they support it or not. This change means that in that situation, we only build projects that are configured to cross build against a Scala version that is binary compatible with 2.11.5.

Of course, another way to achieve this would be to first switch to a dummy project that only aggregated projects that support that version, but this negates that need to have such a dummy project.

Of course, this doesn't work if you:

++2.11.5
compile

But it's convenient. If a project is specified, then it builds it regardless of the support - perhaps we could make that an error though.

[eed3si9n]

This reminds me of similar idea I had, which is stricter version of ++ that only changes the Scala version only if it's listed in crossScalaVersions. This won't allow you to experiment with random version, but it'd be safer behavior during cross building.

Should we make a new operator for that like +++?

[jroper]

Would the new operator be the stricter one, or the one that let you experiment?

[eed3si9n]

If sbt-doge implementation eventually merges to sbt 1.0 or something it might be better to keep ++ the experimental one, and +++ or something the stricter one? I am open to ideas here.

[jroper]

Sounds fine to me.

[eed3si9n]

Tentatively we can call it omg operator.

[jroper]

I called it the plz operator, and I also made it switch back to the old scala versions after running.

[eed3si9n]

awesome