Custom headers

Add the (undocumented) ability to specify custom headers that get sent along with the requests `clever-js` makes.
I have no idea. Until had the need to support custom headers I wouldn't have been able to name that as an option we might have wanted. 

One thing we could "hypothetically" want to support is the ability to turn off certificate pinning to allow hitting non `api.clever.com` domains. 

Like say we don't have any specific other options now but I think it's unreasonable to expect we'd never think of any other options to add.