Add function to render a hyperlink

It might seem cleaner to integrate this into the text style system, so that you
could say `ts := ts.Url("some link")` and then `ts.Sprint("my text")`. However,
this would require adding a new field to TextStyle, which I didn't want to do.

pkg/gui/style/hyperlink.go

@@ -0,0 +1,13 @@
+package style
+
+import "fmt"
+
+// Render the given text as an OSC 8 hyperlink
+func PrintHyperlink(text string, link string) string {
+	return fmt.Sprintf("\033]8;;%s\033\\%s\033]8;;\033\\", link, text)
+}
+
+// Render a link where the text is the same as a link
+func PrintSimpleHyperlink(link string) string {
+	return fmt.Sprintf("\033]8;;%s\033\\%s\033]8;;\033\\", link, link)
+}