Output directory argument

[edemaine]

Justin requested a -o argument to specify the directory that generated outputs should go, to keep source files cleanly separated from outputs. xlsx files do tend to generate a lot of image files.

Should there be a separate setting for .svg and .pdfs? Maybe not necessary, though I have seen some repositories organized that way.

Image paths might be a bit messy to do until we have <image> inlining (#42).