Highlight status bar item when using hot key

We have to use a custom view for the NSStatusItem.
I've tried to match the default style as closely as possible.

The code is heavily based on a blog post [1] by Kris Johnson.

[1]: http://undefinedvalue.com/2009/07/07/adding-custom-view-nsstatusitem
Remove redundant type cast
Use an implicitly unwrapped optional
Allow config to return nil
Display an error message if the config file can't be parsed
Handle errors when the config file can't be read
Prefer camel case