I got the following error when running `checkout-head-revision`:
```
Uncaught TypeError: undefined is not a function /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:804
addCommandListeners.editor:checkout-head-revision /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:804
(anonymous function) /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:840
jQuery.event.dispatch /Applications/Atom.app/Contents/Resources/app/node_modules/space-pen/vendor/jquery.js:4676
elemData.handle /Applications/Atom.app/Contents/Resources/app/node_modules/space-pen/vendor/jquery.js:4360
module.exports.KeymapManager.dispatchCommandEvent /Applications/Atom.app/Contents/Resources/app/node_modules/atom-keymap/lib/keymap-manager.js:400
module.exports.KeymapManager.handleKeyboardEvent /Applications/Atom.app/Contents/Resources/app/node_modules/atom-keymap/lib/keymap-manager.js:181
module.exports.WindowEventHandler.onKeydown
```
It looks like it was caused by a bad merge conflict resolution in 0eaec57.
Test Plan:
Built new Atom and pressed `cmd-alt-z` and didn't get an error.
Looked at a side-by-side diff to check for any other merge issues:
```
git difftool -t opendiff 32e59ce..32e59ce^ src/editor-component.coffee
```
coffeelint.json had some rules set to ignore, but since #3116 was merged there
are no violations of these rules in the code anymore. Tighten up the rules to
prevent those kinds of errors from creeping back in.
Test Plan:
Lint executes without finding any errors
```
$ ./script/grunt coffeelint
Running "coffeelint:src" (coffeelint) task
>> 80 files lint free.
Running "coffeelint:build" (coffeelint) task
>> 23 files lint free.
Running "coffeelint:test" (coffeelint) task
>> 40 files lint free.
Done, without errors.
```
This moves observation of the config keys to Editor, which assigns the
invisibles hash or null on the TokenizedBuffer via the DisplayBuffer to
control whether we render invisibles or not.
This commit also extends config.get to merge default values into the
returned object if both the assigned and default values are objects.
This allows 'atom.invisibles' to be treated as an object that always has
the default values filled in.
