Add metaKey to markdown-textarea, for macos

On MacOS, Ctrl is less commonly used than Command (⌘).
Javascript expresses command as `metaKey`.

This allows for _either_ Ctrl or Command to be used in the Markdown text
area.

Note that on Windows, on some browsers, the "windows" key is labeled as meta, so it would
work on windows as well.

http://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/metaKey

src/shared/components/common/markdown-textarea.tsx

@@ -473,7 +473,7 @@ export class MarkdownTextArea extends Component<
   // Keybind handler
   // Keybinds inspired by github comment area
   handleKeyBinds(i: MarkdownTextArea, event: KeyboardEvent) {
-    if (event.ctrlKey) {
+    if (event.ctrlKey || event.metaKey) {
       switch (event.key) {
         case "k": {
           i.handleInsertLink(i, event);