Add support for in-press and no date sorting and grouping config

[bdarcus]

Probably the cleanest is to add these options to the SortKey enum, so one can just do:

sort:
  template:
    - author
    - in-press # or status?
    - no-date
    - year

Another option is:

sort:
  noDate: 0
  inPress: 3000

But need to also incorporate grouping here, so the decisions should well-accommodate both.

It's not only that the year suffixes need to be correct, when needed, but also need to leave room for things like reference list sections based on these (think a CV).

See also #76.