Output based on tag hierarchy instead of a flat file structure

[TehShrike]

It would be useful to me if, instead of dumping all markdown files to the same output directory, they could be output based on tags.

So if a document had the #cool and #fancy/pants tags, the file would be written to both the cool directory, and the pants directory inside the fancy directory.

Documents without a tag could be written either to the root output directory, or an untagged directory. Without having thought about it a lot yet I'm leaning towards untagged.

This feature should probably be enabled by a new argument, maybe --use-tags-as-directories or something. mri could be used for easiness.