Add permalink configuration for non-zero padded dates

[look]

This adds new permalink configuration to allow non-zero padded months (:monthnopad) and days (:daynopad) to be used in URLs. For example, 2018-01-06 can be rendered as /2018/1/6.

See: https://discourse.gohugo.io/t/implementing-additional-date-formats-for-permalinks/17860

I also looked into implementing this using strftime escapes, as hinted at by this TODO comment introduced way back in 07978e4:

hugo/resources/page/permalinks.go

Lines 217 to 218 in cafecca

	//TODO: support classic strftime escapes too
	// (and pass those through despite not being in the map)

I did find a very nice pure Go strftime library. However, it wasn't clear to me how to fit stftime escapes into the permalink configuration that exists today, so I went with the straightforward solution of adding new configuration.

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[bep]

I also looked into implementing this using strftime escapes,

That comment is very old, and I'm not sure how sensible it was.

What would probably make more sense was to use the date format built into Go, e.g. 2006-01-02

Adding the daynopad etc. feels a little off; I guess it's OK if this is it, so to speak. But we could maybe also consider using the Go date constants, e.g /2006/1/2 etc.

[look]

Thanks for the quick feedback.

I'm in favor of providing a way for users to specify their own date format for permalinks, that feels the best to me. And given that the Go community really does not use strftime much, using the standard library's date formatting seems reasonable. 👍

I'd be happy to work on that. However, I'm not sure how to configure that in the permalinks entry. Do you have ideas?

This looks a little weird, but would match the attributeRegexp and then could be passed along for further processing.

permalinks:
  posts: /:format=2006/:format=1/:format=2/:slug

Or perhaps the config could optionally become a map (not sure how hard that would be to implement) to make the format string unambiguous.

permalinks:
  posts:
    format: /2016/1/2/:slug
  pages: /:title

Thoughts?

[bep]

I was more thinking in the line of making "numbers" the special case, e.g.

permalinks:
  posts: /2006/1/2/:slug

[look]

Ah yes, that makes sense. I will try that and see how it works out.

Do you think it will impact anyone who might be using a literal number in their permalinks? It would only be those that are for Go's special case date, I suppose.

[bep]

Do you think it will impact anyone who might be using a literal number in their permalinks?

I didn't think about those ...

We could do:

permalinks:
  posts: /:2006/:1/:2/:slug

The time.Format looks like it's doing what we want:

https://play.golang.org/p/DHSLL0h_cLk

[look]

I like that. Giving it a try now.

[look]

Closed in favor of #6489.

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