update.py: Allow to output pretty (human-readable) index.xml .
update.py: Allow to output pretty (human-readable) index.xml .

fantastico/fdroidserver,pfalcon/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,OneEducation/AppUniverse_Server,matlink/fdroidserver,pfalcon/fdroidserver,f-droid/fdroidserver,pfalcon/fdroidserver,fdroidtravis/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroidserver,matlink/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,matlink/fdroidserver,f-droid/fdroid-server,fantastico/fdroidserver,fdroidtravis/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroid-server,f-droid/fdroid-server,matlink/fdroidserver,fantastico/fdroidserver,matlink/fdroidserver,fantastico/fdroidserver,fantastico/fdroidserver,pfalcon/fdroidserver,pfalcon/fdroidserver,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroidserver