Removed text substitution from hyperlinks

This was done because handling text substitution would make rich text
renderers much more complex. Instead of lazily preprocessing all
segments into a list of attributes and lazily inserting them into a
new text, they would now have to account for text substitutions,
which would be overkill for a single link.

As a tradeoff, frontends that don't render rich texts and only use the
Content string will not see any URLs. Instead, it will only see the
underlying text of the URL, except without the actual hyperlink.

text/text.go

@@ -24,7 +24,7 @@ type Segment interface {
 // tag with href being the URL and the inner text being the text string.
 type Linker interface {
 	Segment
-	Link() (text, url string)
+	Link() (url string)
 }
 
 // Imager implies the segment should be replaced with a (possibly inlined)