Add check for truncatechar (#420)

* Add check for truncatechar

When value is longer than 1 character, it throws an error

* Use runeSliceWidth instead of len

eval.go

@@ -315,6 +315,11 @@ func (e *setExpr) eval(app *app, args []string) {
 	case "timefmt":
 		gOpts.timefmt = e.val
 	case "truncatechar":
+		if runeSliceWidth([]rune(e.val)) > 1 {
+			app.ui.echoerr("truncatechar: value should be 1 character long")
+			return
+		}
+
 		gOpts.truncatechar = e.val
 	default:
 		app.ui.echoerrf("unknown option: %s", e.opt)