sbt 1.x support

[blast-hardcheese]

There didn't seem to be many changes needed to get this up and running with sbt 1.x, and I've been able to use it locally without any issues.

I'm still new to sbt plugin development, so there may be some considerations here that I'm not aware of yet. In particular, I found:

• sbt-compat which seems to only be for easing the transition to the new structure
• sbt-1.x-plugin-migration which seems to be tracking plugins that have successfully upgraded to sbt 1.x
• Cross Build Plugins from the official docs, but I don't know how to set that up in a safe way that won't confuse the build process.

If you've got some pointers, I'd appreciate them -- I wouldn't want to leave 0.13 hamstrung by this upgrade

[lightbend-cla-validator]

Hi @blast-hardcheese,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it. Please review the new CLA and sign it before we proceed with reviewing this pull request:

http://www.lightbend.com/contribute/cla