Add support for single quotes in links URL

[zhouzi]

Right now, the following Markdown:

This is an [example](https://example.com/link's_example).

Is parsed to:

This is an [example](<link href="https://example.com/link's_example">https://example.com/link's_example</link>)

Instead of:

This is an <link href="https://example.com/link's_example">example</link>.

GitHub supports it, as well as other Markdown editors I played with.