feat(pause): add the browser.pause method to enter a webdriver-specific debugger

Warning: this is still beta, there may be issues.
Usage: In test code, insert a `browser.pause()` statement. This will stop
the test at that point in the webdriver control flow. No need to change the
command line you use to start the test. Once paused, you can step forward,
pausing before each webdriver command, and interact with the browser.
Exit the debugger to continue the tests.