added support for extra/custom query params with test

[btimo]

Solution for issue #315.

Spec are as followed:

• only accepting object
• default value is {}
• can set initial value
• can get value
• can set new value
  • if value not an object, set to default value {}
• (key,value) couples are added to the end of the url

[brockallen]

Looks pretty good -- I'll merge when I can sit down and really go over it. Thanks!

[btimo]

UP 🙃

[skolmer]

This will be really useful to us, thanks. 👍 How about allowing this on the signout request too?

[brockallen]

Thanks!