This patch:
- adds experimental "transport" option to pptr.connect
- uses "transport" option to make sure Puppeteer-Web works with
Target.exposeDevToolsProtocol
Drive-by: add `browser.target()` to access browser target.
This patch:
- introduces a transpiler which substitutes async/await logic with
generators.
- starts using the transpiler to generate a node6-compatible version of puppeteer
- introduces a runtime-check to decide which version of code to use
Fixes#316.
