Consider making `java_import` targets executable 

Some JARs are "executable" in the sense that they can be run directly with `java -jar`. This is supported by setting a main class in the JAR's manifest file.

It may be convenient to be able to run such JARs using `bazel run`. The `java_import` rule could support this. An `executable` boolean attribute could be added to the rule to let the user indicate whether a JAR is in fact executable.