Can now not just handle an exact year field (with only the year
contained), but also the biblatex supported 'date' field from which it
will extract any 4-digit number as the year provided.
Options now reside under the 'plugins.bbt' section (instead of
'plugins.bbt-formatter'). The 'fallback' option has been renamed
'default-formatter' to signify its purpose a little better - even if it
will be rarely changed in all likelihood.
Whereas before everything would be written in the `ref-format` string,
we can now simply put the customization options like any others passed
to papis - through its config file.
Comes with default options. Explanation added in README.
