Allow YYYY-MM-DD-HH-MM-SS prefix when extracting date from file names.
This is in addition to the YYYY-MM-DD prefix that is already supported.
Closes#13830
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
