Date Formats

- Fixed broken documentation for custom report definitions regarding column
  formats.
- Modified date access to use the more efficient Date::get_date method, which
  also shrinks the code.
- Added the 'remaining' format for all date columns.
Bug #1248

- #1248 Merge tests no longer connect to takwarrior.org (thank to Jakub Wilk).
Bug #1304

- Made the 'task shell' replacement by 'taskѕh' more prominent in the NEWS file.
- Added default alias 'alias.shell=exec tasksh' for continuity.
Merge branch '2.3.0' of tasktools.org:task into 2.3.0