Allow negative numbers to be passed to parseHeight function

Author: André Kremser

src/gridstack.js

@@ -99,7 +99,7 @@
             var height = val;
             var heightUnit = 'px';
             if (height && _.isString(height)) {
-                var match = height.match(/^([0-9]*\.[0-9]+|[0-9]+)(px|em|rem|vh|vw)?$/);
+                var match = height.match(/^((-[0-9]+\.[0-9]+|[0-9]*\.[0-9]+)?|-?[0-9]+)(px|em|rem|vh|vw)?$/);
                 if (!match) {
                     throw new Error('Invalid height');
                 }