Sort attributes alphabetically

[gligoran]

Hi!

Firstly, thanks for this simple, little extension. I'm using it for an Ionic2 project where an XML gets generated when running certain commands. For some reason it's always generated a bit differently and makes a mess out of commits. The biggest difference seems to be the order of attributes. It would be nice if the formatter would consistently order these attributes the same way, so I could format the file if there are any changes and get a clearer view of what actually changed.