Add a plugin for querying date fields against date intervals or instants...

A plugin for querying date fields against date intervals or date instants.

The interval syntax is similar to that of NumericQuery: beet ls 'added:T2008..2010'

This plugin is intended to be used together with the [importmtimes](https://github.com/sampsyo/beets/pull/520) as it lets one query the data that 'importmtimes' imports.

I would really like to see date interval based queries as part of Beets core, but for now I have implemented it as a plugin.

Please review the code and let me know if anything should be changed.