Improve perf by removing a loop that does IO
Add support for reading from sitemap.xml
Add support for an external spidering skip list
Display paths properly in gallery

* use __ rather than _ as the ‘dummy’ separator char,
  as it is much less commonly used than _ in a
  file/directory name
* replace __ with / when displaying paths in the gallery
Add a mode flag, allows sorting/hiding by difference size

Fixes #153

Conflicts:
	configs/config.yaml