Add keyboard shortcuts for clearing the console

Summary:
This adds a keyboard shortcut for clearing the console that matches Chrome's. A few things to note:

1. It's global (instead of being scoped to the input prompt).
2. Even though it's preferable, we don't use ⌘K because of its conflicts with partially matching pane moving shortcuts. Also, it's currently broken due to atom/atom-keymap#129.

Changelog: Add keyboard shortcut for clearing the console

Reviewed By: jgebhardt

Differential Revision: D3825846

fbshipit-source-id: e9970739ecf9fd78b6b2a87449503c20410aa2cc

pkg/nuclide-console/keymaps/nuclide-console.json

@@ -4,5 +4,11 @@
   },
   ".platform-win32 atom-workspace, .platform-linux atom-workspace": {
     "ctrl-shift-j": "nuclide-console:toggle"
+  },
+  ".platform-darwin atom-workspace": {
+    "ctrl-l": "nuclide-console:clear"
+  },
+  ".platform-win32 atom-workspace, .platform-linux atom-workspace": {
+    "ctrl-l": "nuclide-console:clear"
   }
 }