This change introduces a PUPPETEER_PRODUCT environment
variable as a first step toward using Puppeteer with
many different browsers. Setting PUPPETEER_PRODUCT=firefox, for
example, enables Firefox-specific Launcher settings.

The state is also exposed as `puppeteer.product` in the API
to support adding other product-specific behaviour as needed.

The bulk of the change is a refactoring in Launcher
to decouple generic browser start-up from product-specific
configuration.

The funit script is renamed to fjunit (j for Juggler, which is
used only by the experimental puppeteer-firefox package.

In contrast, the funit script now refers to running Puppeteer
unit tests against the main puppeteer package with Firefox.
To do so with Firefox Nightly, run:

`BINARY=path/to/firefox npm run funit`

A number of changes in this patch make it easier to run
Puppeteer unit tests in Mozilla's CI.

Respecting the puppeteer-core restriction for PUPPETEER_
environment variables, lazily instantiate the Launcher
based on a `product` Puppeteer.launch option, if available.