Add support for time.Time flags

[max-frank]

This PR implements support for time.Time flags which can be used to accept timestamps as input directly (see https://github.com/spf13/cobra/issues/742).

The implementation allows defining a list of acceptable timestamp formats which are tested in order, i.e., devs using this flag type would be responsible for ordering their formats correctly should there be an overlap.

[kostrahb]

This is exactly what I am looking for. Would it be possible to review and merge this?

Maybe also suggestion for small improvement, would it be possible to offer some reasonable standard default ordering? Like this:

var DefaultOrdering []string = []string{time.RFC3339, time.UnixDate, ...}

or this:

func getDefaultOrdering() []string {
	return []string{time.RFC3339, time.UnixDate, ...}
}

[max-frank]

I was thinking of adding a constant slice with a suggested default time format set, but opted to not include it in this PR since its easy to define such a constant in whatever code you use and more importantly I wanted to avoid any long winded discussions on what the actual order and included formats should be.

[max-frank]

@johnSchnake tagging you (a bit late) for this PR as requested in https://github.com/spf13/cobra/issues/742

[alexferrari88]

hate being that guy...but: any news on when will this be in?

[max-frank]

Yeah sadly seems that the repo is just sitting here now anymore nothing happening :/

i tried to make some noise in the related issue on the cobra side but nothing yet

[mmontes11]

Anything needed from the community side to cross the finish line with this PR? Happy to test, review, anything... it is a bit of a shame that the de-facto standard CLI library doesn't have a time.Time flag like it has already for time.Duration

[Kapparina]

Good to see this has been approved... Can it be merged?

[max-frank]

Good to see this has been approved... Can it be merged?

@Kapparina i wish it was this easy :(