Add content-normal and content-stretch utilities

[dcastil]

I'd like to add a utility for the CSS align-content: stretch.

This is the default align-content value for flexbox and grid layouts if none other is specified. The reason to include this in Tailwind is that it is necessary to be able to reset the align-content value to its default on a breakpoint, e.g. content-center lg:content-stretch. Also it isn't possible to use arbitrary values for align-content since content-[stretch] would instead create the CSS content: stretch. So the only option to use that CSS currently is with arbitrary properties: content-center lg:[align-content:stretch].

Let me know what you think. 😊

[reinink]

Hey thanks for this! I think this makes sense to add. I've also added content-normal with this update 👍