Adds just::run_with_args to provide arguments from Rust code

[EnigmaCurry]

This adds a new method to run just commands passed from a Vec<&str>.

I found this blog post where I copied this pattern for optional arguments, I didn't want to mess with the original arity of the run() method, so I made a second method for passing args, and a third to consolidate the common logic, but maybe there's a better way?

This example runs the foo target:

cargo run --example run-with-args
 ## (stdout): bar

I don't quite know how to test this yet either, because I'm not forking the process, I don't think I can capture stdout like the other tests do.

[casey]

Thanks for the PR!

I think it's fine to just have a single run() which takes arguments, and pass in std::env::args_os() from main. Also, this is small enough that I don't think it needs to be tested. I merged #2173 which does this.

A couple things to keep in mind when using just as a library:

• Since just is primarily a CLI tool, the library interface doesn't have any kind of semantic versioning guarantees, and may wind up breaking or changing.

• Also, run() will not return if the command line arguments can't be parsed, since currently we call get_matches_from() which will exit on error. This could be changed without too much work, and make run() return an error instead of exiting when the args can't be parsed.

I added docs to this effect in #2174.

[EnigmaCurry]

Thanks!