Add toggle commands

This will let people override the `cmd-f` and `cmd-shift-F` keybindings to toggle the find-and-replace views. Just add the following to `~/.atom/keymap.cson`:

```coffeescript
'.workspace':
  'cmd-f': 'find-and-replace:toggle'
  'cmd-shift-F': 'project-find:toggle'
```

Closes #161 
Will make #198 and #134 :smile: