Allow extra command line arguments #297

[FlashSheridan]

• Pass, per the maintainer’s suggestion, extra arguments to the child subprocess, so that they can be seen in the REPL.
• Add, for Rust newbies like me, a new command-line option to allow more command-line arguments:

--extra-args <extra-args>... Extra arguments; ignored, but show up in std::env::args()

I am, by hypothesis, a Rust newbie, so review may be in order.

Testing

• EVCXR_TMPDIR=$HOME/tmp/e1 cargo test -- --test-threads 1 passed on MacOS 12.6.2 21G320 on M1 Max, with the same two crash dialogs I got from main.
• target/debug/evcxr --opt 2 --edit-mode emacs --extra-args \\--foo bar baz gave:

>> std::env::args()

Args { inner: ["/Users/flash/Documents/Code/external_code/evcxr/target/debug/evcxr", "target/debug/evcxr", "--opt", "2", "--edit-mode", "emacs", "--extra-args", "\\--foo", "bar", "baz"] }

[davidlattimore]

Thanks!