Parse config options using getopts as well

[CohenArthur]

https://github.com/Rust-GCC/cargo-gccrs/pull/5#discussion_r649516391

[CohenArthur]

I think having some sort of 'callback' would also be useful in order to compile static libraries. Since we need to spawn a second command to archive the object files into a singular .a file, we could do this in said callback.