rdmd: Add --dependfile option

The idea behind this option is to provide something similar to GCC's -MF
option, which generate a dependency file at the same time it compiles
the program. This is very useful to keep a Makefile with the
dependencies always updated:
```make
-include .deps.mak

prog:
	rdmd --build-only --dependfile=.deps.mak prog.d
```
On the first build .deps.mak will be created and afterwards, the only
way the dependencies could change is if prog.d or any of its
dependencies change, in which case a build will be triggered and the new
dependencies file will be generated.