Add support for overriding ajax calls with custom transport methods

**Context**

> The Internet Archive's Distributed Web project (https://dweb.me) uses different protocols than just http / https, including WebTorrent and ipfs. This means we need a method other than using a standard Ajax requests for it to work in the Dweb version.

**Proposal & Constraints**

> When we initialize BookReader, we pass in an options object with configuration options. We could potentially add a configuration option to let the consumer define a function that will be used in place of the ajax requests. NOTE This is just an idea and will need to be prototyped for feasibility.

**Success Metrics**

> If Mitra is able to integrate the BookReader into the IA Decentralized Website, this is successful.

**Stakeholders**

> @mitra42