Add mechanism to disable POM-reading for specific subprojects in multi-project builds

[JoshRosen]

In a multi-project build, I would like to be able to disable the sbt-pom-reader plugin for specific subprojects because those projects do not have POM files for their dependencies. In other words, in a multi-project build I would like to be able to define some subprojects using libraryDependencies and have others load their dependencies via the POM reader plugin.

This is motivated by a use-case in Spark.

[metasim]

@JoshRosen Would merging #14 take care of this issue?

[JoshRosen]

I think that issue is unrelated.

[jvican]

This can be solved with .disablePlugins(com.typesafe.sbt.pom.PomReaderPlugin).