Add support for extra settings and custom source location

[mone]

Working on a project where I need to run antlr against the same grammar in 2 different sub-projects, 1 being a normal scala project the other being a scala.js project. For the latter I also need to add an extra parameter to let antlr generate javascript code: I figured it would make sense to add a generic extra parameters setting for these kind of corner cases rather than add a direct mapping to the -language flag

I noticed there are a couple of open and abandoned PRs, is the whole repo abandoned?