Make a -package foo command line option

[parrt]

We sometimes need to know if the generated code is going into a package or whatever the target language called package. For example, when generating interfaces and such, it would be nice to say

import static MyParser.*;

so that we can simply reference FooContext instead of having to prefix it with MyParser. If there is no package specification that we generate in the default package. Due to a limitation in Java, we must prefix the context names with the parser name.

[sharwell]

This option was initially added in 913bb71, then tweaked in the following to a working state:

71276f7
8e3b85d