Using snippet options object for FullStory.init

[patrick-fs]

I've refactored the parameterized function signature for FullStory.init to use an options argument. This also applies to the snippet function as well.

Additional changes:

• added _fs_script to snippet - this reflects the latest change to the snippet made by the RiR team
• removed the fs prefix from snippet option key names
• throwing an error if orgId is not provided as a snippet option and added a test
• updated tests; now cleaning up the window object before each test to return to pre-initialized state