Custom headers

Add the (undocumented) ability to specify custom headers that get sent along with the requests `clever-js` makes.
My reasoning behind this is there is a good chance we'll want more options at some point in the future. If this comes up the format will probably be `Clever(auth, path, options)` not `Clever(auth, path, headers, options)`. In that case I'd rather design it that way from the beginning as opposed to forcing us to change how we consume this library everywhere when we make that switch.