Add support for typed __version__ substitution.

[da2ce7]

• Includes basic reformatting of the substitution section in the readme.

[mtkennerly]

Hi! Thank you for the PR, and I apologize for the late reply. However, there are a couple of issues here:

• str is correct, but I don't think it's necessary to be so explicitly restrictive. I'd rather just use something like .*?. Someone could use some slightly different formatting or a type alias, and although it would be unusual, I don't think there's any particular reason to exclude it in the regex.
• I prefer the original formatting for the README. I think it's more consistent with the rest of the document.

[da2ce7]

Hello @mtkennerly,

You have closed this pull request, would you like me to try and address your comments. Or are you planning to include support for the typed version substitution on your own?

[mtkennerly]

I'd be happy to accept the PR if you'd still like to address the comments :) I just wasn't sure how soon you'd see this since I replied late.

[mtkennerly]

Closing in favor of b40453b.

[da2ce7]

@mtkennerly thank you, just now I was meaning to get around to addressing the comments and now see you have fixed it already 👍.