nteract: URLs with spaces do not render a links in markdown cells

I love regex.

However, we have to take a step back here to understand and resolve the core problem while assisting the user as much as possible. Spaces have a specific meaning in Markdown URLs both in CommonMark and the original Markdown “spec”:

To create an inline link, use a set of regular parentheses immediately after the link text’s closing square bracket. Inside the parentheses, put the URL where you want the link to point, along with an optional title for the link, surrounded in quotes. For example:

This is [an example](http://example.com/ "Title") inline link.

[This link](http://example.net/) has no title attribute.

A URL with spaces is not a valid URL. The URL with %20 is URL encoded, quite valid. There is an important reason to support the title attribute - for accessibility and screen readers. This becomes more important when doing links on images from Markdown.