Allow replacing default service worker's url - fix #19

[mdibaiee]

Hey, I really like the idea of UpUp, I'll try to keep contributing as much as possible ✊

[mdibaiee]

@TalAter any feedback on this?

[TalAter]

Thank you @mdibaiee! Merged 👍

Sorry it took me a while, I was travelling abroad for 3 months.

A couple of comments:

• I had to change UpUp.addSettings(). When you added service-worker-url to the list of settings, it broke UpUp if users relied on its default and didn't pass it a service-worker-url (it would overwrite the default url with null) -

  UpUp/src/upup.js

  Line 169 in 7130d92

  if (settings[settingName] !== undefined) {

• The API docs are generated automatically using markdox. In your next PR (I hope there are more 😄), instead of changing docs/README.md directly, change the comments in the upup.js file... Otherwise your changes will be overwritten automatically when the docs are generated.