Added a switch to enable autolinking of links.

[appel]

As discussed in #326. It's not pretty but it seems to work.

[edent]

I like this, but I think it breaks if there's a link inside another link.

For example:

Markdown [Visit https://example.com](https://example.com)
HTML <a href="https://example.com">Visit https://example.com</a>

Is rendered as:

<a href="https://example.com">Visit <a href="https://example.com">https://example.com</a></a>

[appel]

Hey edent, thanks for your comment. Honestly, I can't remember if I tested this at the time, but I agree that that would be a bit of an issue. :) I don't think my PR really went anywhere and I have since moved on to a different MD implementation, so I won't be able to look into this, unfortunately.