add stat-based include/exclude

there are tickets about doing file size based exclusion: #902, jborg/attic#330

file size is a stat result attribute, so this is a special case of a stat-based rule.

also in stat result:
- timestamps: atime, ctime, mtime
- type and mode
- uid / gid

so we could add a mechanism to define inclusion / exclusion rules not only based on the file's path/name (as we already have), but also based on comparing stat attributes to given values.