Allow paired files to be by ext

[gtronset]

This adds the ability to be specific about which extensions should qualify as being a "pair", allowing for targeted filetypes instead of having to rely upon the global Filetote extension list.

These extensions are additive and independent from the global list, meaning those specified in the pairing config node will apply even if they do not otherwise appear in the global list. This can be useful for only targeting paired files, or only caring about certain file types if they are pairs.

This introduces a breaking change to the config of pairs. pairing has been converted from a boolean to an object with other like-config. Take the following config:

filetote:
  pairing: true
  pairing_only: false

These will both now be represented as individual settings within pairing:

filetote:
  pairing:
    enabled: true
    pairing_only: false
    extensions: ".lrc"

Both remain optional and both default to false.