Add "Copy cell" to ManagedTable

Summary:
Previously you could only copy the entire row.
Now you can choose which cell to copy.
Not the perfect UI admittedly, but that would require a big change to make individual cells selectable. I think it's a definite improvement.

Reviewed By: passy

Differential Revision: D15231784

fbshipit-source-id: 0d52192a5bf4e8ea1ebee74988c749d3f602fea9

flow-typed/electron-menu.js

@@ -53,7 +53,7 @@ type Electron$MenuItemOptions = {
   enabled?: boolean,
   visible?: boolean,
   checked?: boolean,
-  submenu?: Electron$MenuItem | Electron$MenuItemOptions,
+  submenu?: Electron$Menu | Array<Electron$MenuItemOptions>,
   id?: string,
   position?: string,
 };