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, ".di" is a groff macro and I can't find how to escape it. Does anyone knows?