2.12 support

[cvogt]

@SethTisue implemented support for 2.12.0 https://github.com/cvogt/browse/tree/topic/2.12-compat

I added support for 2.12.1 https://github.com/cvogt/browse/tree/topic/2.12.1-compat

Looks like @paulp had done this, too, but I can't find the source code. The binaries are here:
https://bintray.com/paulp/maven/sxr/1.0.2#files/org/improving/sxr_2.12/1.0.2

@paulp Since this is a compiler plugin, it should be released with scala minor versions in the artifactId. The plugin is definitely binary incompatible between 2.12.0 and 2.12.1 as you can see from my patch here: cvogt@67261cd (the previously overridden method became final in 2.12.1 and the new one didn't exist in 2.12.0)

[cvogt]

cc @dwijnand @retronym @eed3si9n

[cvogt]

cc @OlegYch @lrytz
(just cc-ing some people who worked on this or sbt, so we don't replicate each others work)

[eed3si9n]

There's an unmaintained fork called sbt/sxr for sbt's use, and Paul forked that as https://github.com/paulp/sxr.

[eed3si9n]

never mind there's already a conversation going on in #26