API and test improvements

[mgechev]

Most of the changes are related to refactoring and the introduced e2e tests.

This PR introduces the following changes:

• API improvements as discussed in [RFC] Guess function API #42
• Externalizes the guess method for predictions so we can later publish it as a separate package and reuse it across packages
• Introduces infrastructure for e2e testing with puppeteer
• Makes the payload smaller for apps which are going to be consumers of guess only (i.e., won't need prefetching functionality on route change, such as Gatsby)
• Makes the route extraction API asynchronous

@KyleAMathews @khempenius would you share your opinion about the current API? How convenient it's going to be for Gatsby and would we be able to reuse it in the static page solution (look at packages/webpack/src/runtime/guess.ts)?

Fix #42.

[addyosmani]

After reading through the commits, I think on the whole the API changes here are reasonable. I didn't have any further concrete feedback to share.