Add command line parameter to allow customization of URL in QR code

[jonek]

In certain situations it is not possible to reach the URL that is encoded in the QR code from a mobile device. In this situations it would be handy to be able to specify a custom URL when executing tabris serve -l.

For example: tabris serve -l --url http://custom-url:1234

[cookieguru]

--external option added in 2c0de40