Add a mode flag, allows sorting/hiding by difference size

Fixes #153

Based on discussions in #153 I have implemented 3 modes for wraith, which are described in the sample config.yaml file. I believe this meets the requirements described there:

1) Allow display of differences first
2) Describe them as differences/diffs, not failures
3) Allow this mode to be toggled on/off (existing behaviour is maintained)
4) Allow only diffs to be recorded (both in the gallery, and on disk)
5) Keep only one copy of a file around