rdmd: Add --extra-file option

This option includes an extra source or object in the compilation. It is
particularly useful if you need to add an extra object (compiled by
another language maybe) or an extra source (maybe because a .di was
provided) when compiling the program (multiple --extra-file are
allowed).
Mmm, that looks inaccurate for me. The problem is if you import a module for which a `.di` was found, then rdmd won't compile the corresponding `.d` file (even if present) as part of the program. If you know a better way to word that in a succinct way, I'm all for it :)