Text format string for linkifiers

[varkor]

In the Category Theory Zulip server, we're using a linkifier to turn [[text]] into a hyperlink to the corresponding nLab page. This works really well. The only downside is that linkifiers turn the entire matched string into a hyperlink so that we get links that look like [[text]], which is a bit ugly. It would be nice if, as well as a URL format string, we also had a column for Text format string, which would allow us to customise the text for the hyperlink (making use of the regex captures), so that [[text]] could display as text.