Selectable filetype-sorting strategies

[xandkar]

• different subtrees of same root: $root/{img,vid}/$year/$month/$day/$file. [current default]
• unified: $root/$year/$month/$day/[$img_file | $vid_file]
• filtered: each run specifies which filetype to look for, so either of the above strategies can be achieved with 2 separate executions [old default]