Add italics support for truecolour terminals

Author: Ray Lee

formatters/tty_truecolour.go

@@ -21,6 +21,9 @@ func trueColourFormatter(w io.Writer, style *chroma.Style, it chroma.Iterator) e
 			if entry.Underline == chroma.Yes {
 				out += "\033[4m"
 			}
+			if entry.Italic == chroma.Yes {
+				out += "\033[3m"
+			}
 			if entry.Colour.IsSet() {
 				out += fmt.Sprintf("\033[38;2;%d;%d;%dm", entry.Colour.Red(), entry.Colour.Green(), entry.Colour.Blue())
 			}