File stat (e.g., mtime) timestamps

[shtrom]

This introduces a modular way to extract dates from files. This is useful to use the mtime stat instead of parsing the filename, as mentioned in #19.

The modularity is added by adding a Matcher object, an implementation that use the Filename, and another that uses the Stat information

[shtrom]

@xolox Ok, this PR is good to go. Care to take a look?

Essentially, this adds a --stat-timestamp argument that uses the mtime of the file instead of trying to extract the date from the filename. To do so, I split out the date-extraction logic into a generic interface, so this potentially paves the way to other ways to get the date out.