Make `url` a required option (#105)

[sdvg]

Closes #105

• [x] Make url a required option, adjust tests
• [x] Check if hoodiehq/hoodie tests still work or if they need adjustments too
• [x] default url option in bundle code to location.origin

[sdvg]

This is ready for review now :) Please also see hoodiehq/hoodie#648.