Allow user provided HTTP client

[joepurdy]

Fixes https://github.com/customerio/go-customerio/issues/17

👋 Hey Customer.io Team! Saw an issue filed on the Golang library and decided to take a crack at it. Let me know if there's anything y'all would change about this implementation, at a high level:

• exports the *http.Client property on the CustomerIO struct to allow user customization
• adds an exported DefaultClient var and simplifies the NewCustomerIO() func to use this
• adds a silly test to ensure the Client property of the CustomerIO struct can be modified

@hownowstephen you may be a good candidate to review this or to get it out to someone else from the backend team to review.

[swastik]

Closing this—we're making Client exported via #21.