Create separate permalink option for media files

[pmarsceill]

I host my site's source files in src/, so my media filename env var is:
MICROPUB_MEDIA_FILES_STYLE=\"src/assets/microblog/:year-:month-:day-:filesslug\"

but when the site is compiled, the media files are available via URL at this kind of format:
https://example.com/assets/microblog/:year-:month-:day-:filesslug

The current implementation assumes that these two are equivalent and uses the former in the YAML frontmatter of the post file that it generates, when I would like it to use the latter. It would be nice if there were two options (similar to MICROPUB_FILENAME_STYLE and MICROPUB_PERMALINK_STYLE) for media files... One for the filename (used when uploading media) and one for the permalink (used for referencing the files via URL after the site is deployed).

[keithjgrant]

I would love to have this, too

[voxpelli]

Related: voxpelli/node-format-microformat#3