Add configuration capability and make Google Webmaster Meta optional

A google-site-verification meta tag for Google Webmaster Tools is hard
coded into the code.  While this is appropriate for the public site
at http://dillinger.io/
this key may not be expected behavior for other developers cloning
or using the site.

To fix, a popular configuration library called rc was added.  A basic
starter configuration file for the project was created and the
google-site-verification content option was moved from the hard coded
location in app.js to the config file.
EJS was also adjusted to optionally display this tag as needed.

If desired, the configuration file can also be used for any other
configuration options.