watch: Support ANSI 39,49 reset sequences

ANSI colour sequences 39 and 49 reset the foreground and background
colours respectively. This feature was missing from watch using the
colour "-c" option.

References:
 Fixes #73
 https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters

watch.c

@@ -184,6 +184,12 @@ static int set_ansi_attribute(const int attrib)
 	case 27:	/* unset inversed */
 		attributes &= ~A_REVERSE;
 		break;
+    case 39:
+        fg_col = 0;
+        break;
+    case 49:
+        bg_col = 0;
+        break;
 	default:
 		if (attrib >= 30 && attrib <= 37) {	/* set foreground color */
 			fg_col = attrib - 30 + 1;