* Add `date format --list`, and make format string optional (providing default)
* Make DRY
`into datetime --list` now uses `generate_strfttime_list` from `date format --list`
* refactor strftime to use current datetime
* Fix formatting of specification descriptions
Fixes issues caused when copying directly from docs.rs
* Change default format to rfc2822
Perhaps to make it more DRY, functions from `into datetime` can be used. However, currently `into datetime` is a bit tricky to use as it needs a separate time zone argument.
* Tweak in-shell docs to match modified behavior
* Show %#z format specifier in `into datetime --list` only
* cargo fmt
* Satisfy clippy
