Capture GCLID url param, add option to disable saveParamsReferrerOncePerSession

[djih]

1. Customers have asked for SDK to automatically parse gclid url param (Google click id) and set as a user property (like what we do for UTM params already). Added includeGclid configuration option
2. Currently the SDK will only save referrer and UTM params once per session, at the start of the session. I feel like the logic might be slightly buggy and some customers are reporting that the UTM params are not being captured. Added a saveParamsReferrerOncePerSession option that they can set to false to remove this restriction
3. Before the SDK had this convoluted logic to implement the "once per session" restriction. I saved the UTM parameters and referrer to the browser's sessionStorage. And so if the SDK saw an existing value in the sessionStorage, then it assumed that it already ran the logic and sent the values in the current session. I feel like this could be buggy, and the browser's sessionStorage expiration window and whatever might not line up with our SDK's definition of a session. I removed the use of sessionStorage completely, and the UTM param / referrer saving logic simply just saves the values every time it is called. The init method is the one that decides when to call it depending on configuration options and if a new session is starting. This should make the logic easier to reason about, and give the customer freedom to do whatever they want.
4. Removed a bunch of unnecessary unit tests around the convoluted logic described above. Added new tests for the override option and GCLID.
5. Added a new README section explaining all of these options.

PTAL @ManThursday

[trashstack]

Looks good to me.

[trashstack]

Seems like this line doesn't get used anymore?

[trashstack]

Seems ok but I always get a bit nervous with regex parsing. Is it worth taking into account the possibility of repeated params?