Add support for resources in javadoc jars and make variable substitution in javadoc arguments

[vinnybod]

There's two use cases I'm trying to account for here.

1. I need to add resources to the javadoc jar such as a LICENSE file.
2. I need to send a --define variable to the javadoc generator so that the maven_version variable I have set will appear in the <title> of the generated html. rules_jvm_external already supports the same type of substitution on the coordinates attribute.

java_export(
    name = "lib",
    maven_coordinates="com.example:lib:$(maven_version)",
    ...
    javadocopts=["-windowtitle", "$(maven_version)"],
    doc_resources = ["//:LICENSE"]
)