SI-6963 Add -Xmigration-from switch

We decided to make -Xmigration take an argument. But doing so would
break any build that depends on -Xmigration with no arguments. So
this commit adds -Xmigration-from instead. It's nicely mnemonic.

I considered deferring -Xmigration-from until 2.11, but that would
leave -Xmigration deprecated with only a warning that it will be
removed. Which I guess is technically true, but the full truth is that
it's getting replaced by something better. Adding this switch in 2.10.1
allows the deprecation message to immediately point a user towards what
they should be using instead of just saying BZZT, SCREW YOU BUDDY in
2.10 and then later saying, "oh, you're not that screwed".

This commit adds a ScalaVersion class (plus ancillary stuff), and a
ScalaVersionSetting.

Since @adriaanm convinced me to add a version number, I'll give this to him for review.