Make the output path for compiled manuscripts configurable

[AdamIsrael]

When we integrate with Obsidian, the compilation defaults to using ~/Writing/Drafts as the root of where we output the manuscripts. That should be configurable, so that alternate roots can be selected, i.e., ~/Dropbox/Writing/Drafts.

This will probably look something like:

md2ms obsidian  ~/Library/Mobile\ Documents/iCloud\~md\~obsidian/Documents/loci --output_dir ~/Dropbox/Writing/Drafts

We'll keep the default in place, but offer a path to customizing it without having to edit the shell command(s) by hand.