added zoomKey option to Graph2d, added missing docs

docs/graph2d/index.html

@@ -1026,6 +1026,26 @@ function (option, path) {
             <td>The width of the timeline in pixels or as a percentage.</td>
         </tr>
 
+		<tr>
+      		<td>zoomable</td>
+      		<td>boolean</td>
+      		<td><code>true</code></td>
+      		<td>
+        		Specifies whether the Timeline can be zoomed by pinching or scrolling in the window.
+        		Only applicable when option <code>moveable</code> is set <code><code>true</code></code>.
+      		</td>
+    	</tr>
+
+		<tr>
+		    <td>zoomKey</td>
+		    <td>String</td>
+		    <td><code>''</code></td>
+		    <td>Specifies whether the Timeline is only zoomed when an additional key is down.
+		      	Available values are '' (does not apply), 'altKey', 'ctrlKey', or 'metaKey'.
+		        Only applicable when option <code>moveable</code> is set <code><code>true</code></code>.
+		    </td>
+		</tr>
+
         <tr>
             <td>zoomMax</td>
             <td>Number</td>

lib/timeline/optionsGraph2d.js

@@ -150,6 +150,7 @@ let allOptions = {
   },
   width: {string, number},
   zoomable: {boolean},
+  zoomKey: {string: ['ctrlKey', 'altKey', 'metaKey', '']},
   zoomMax: {number},
   zoomMin: {number},
   __type__: {object}
@@ -251,6 +252,7 @@ let configureOptions = {
     start: '',
     width: '100%',
     zoomable: true,
+    zoomKey: ['ctrlKey', 'altKey', 'metaKey', ''],
     zoomMax: [315360000000000, 10, 315360000000000, 1],
     zoomMin: [10, 10, 315360000000000, 1]
   }