Enable cross building for JVM, Scala.Js and Scala Native

[mkotsbak]

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[propensive]

Thank you, @mkotsbak! I don't have enough SBT skills to review this, but I'm delighted to have it, so I'm merging! :)

I'd also like to publish for Scala JS and Scala Native. Could you tell me the commands to run, please?

[densh]

@propensive Publishing workflow is 1-to-1 the same as on the JVM. As long your typical sonatype/bintray credentials and sbt-pgp signing is configured, myprojectNative/publishSigned does the right thing without you having to configure anything extra. (I think it's the same for Scala.js)

[propensive]

Thanks, @densh! @mkotsbak actually came over to see me at flatMap(Oslo) seconds after I sent that, and we got it published. :)