Support Go time format strings in permalinks
In the vein of an ancient TODO about supporting custom date formatting with
strftime, this allows `:`-prefixed Go time format strings in permalink segments.
This allows users to customize date-based permalinks any way they need to.
For example, with a date of 2019-11-09, the permalink `/:06/:1/:2` will render
as `/19/11/9`.
See:
https://github.com/gohugoio/hugo/commit/
07978e4a4922bc21c230fee65052232b829bd1ab#diff-0688a3b65c7f5d01aa216f8d9b57fd00R111-R112
https://discourse.gohugo.io/t/implementing-additional-date-formats-for-permalinks/17860
https://github.com/gohugoio/hugo/pull/6488