sbt support

[olafurpg]

We have sbt-sourcegraph that does some of the work but it still requires manual configuration and the steps are not documented. Opening this issue to track progress on adding automatic inference.

• Automatically enable semanticdb-javac in sbt-sourcegraph
• Support sbt v1.0-v1.3, or report helpful error message
• Configurable version of SemanticDB and mtags
• Support manual addCompilerPlugin()
• Support a way to ignore pure-crossbuilt projects (JS/Native) #293
• Recover from compile errors in a single project #294

[olafurpg]

Closed in favor of subtasks #293 and #294