`-od` behaves differently than rdmd

[marler8997]

Link to discussion: bee4173

[marler8997]

To summarize, rdmd overrides the behavior of the compiler option -od. Instead of having it set the directory where all object files are stored and the parent directory of the final executable, rdmd only uses it to set the parent directory of the final executable. This means there's no way to tell rdmd where to put the object files (there was an attempt to fix this issue here dlang/tools#350). rund takes the approach of trying to keep the same compiler interface as much as possible, so it's been decided to take the semantics of -od from the compiler rather than use rdmd's varition on it. However, users may find it convenient to be able to set the directory of the final executable without setting the directory for the object files, for this case rund introduces the --od=<dir> option with #6

Normal -od behavior:

# rund -od=. prog.d
# ls
prog.d prog.o prog

New--od behavior:

# rund --od=. prog.d
# ls
prog.d prog