patch 8.0.1038: strike-through text not supported

Problem:    Strike-through text not supported.
Solution:   Add support for the "strikethrough" attribute. (Christian
            Brabandt, Ken Takata)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index d11d96d..1cdb6b6 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -11843,6 +11843,10 @@
 	case 's':
 		if (TOLOWER_ASC(what[1]) == 'p')	/* sp[#] */
 		    p = highlight_color(id, what, modec);
+							/* strikeout */
+		else if (TOLOWER_ASC(what[1]) == 't' &&
+			TOLOWER_ASC(what[2]) == 'r')
+		    p = highlight_has_attr(id, HL_STRIKETHROUGH, modec);
 		else					/* standout */
 		    p = highlight_has_attr(id, HL_STANDOUT, modec);
 		break;