Feature request: add output option

[jonathan-a-thomason]

In many linkers there exists a -o option that allows the ability to specify the name of the output file.
I think this would be a good a addition to the slasm linker, since this also disambiguates what the output name is without needing to worry about the order of the files for the argument list.

For this particular linker it may be possible to append the requirement that the .slb is appended regardless of the chosen name.