Add test for --external option.
Use standard name for --external option internally.

Instead of :external_classes, use :external - like the name of
the option itself, and like it is now with all other options.
Test for overrides getting placed to externals list.
Leave only JavaScript builtins to default externals.

Implement a special '@browser' keyword, which when used as:

    --external=@browser

will include all the standard browser web API classes, listing
of which taken from:

https://developer.mozilla.org/en-US/docs/Web/API

Fixes #502