Support fetching console.log output for Firefox

[naslundx]

Apparently this is not something that's part of the webdriver standard, and mozilla decided not to implement it for geckodriver. So currently running with firefox always fails to obtain JS logs. See: mozilla/geckodriver#284

Relevant:
https://stackoverflow.com/questions/23231931/getting-console-log-output-from-firefox-with-selenium

Geckodriver produces a log file somewhere that should have JS output, we might be able to read and parse that.
Or do this hackish thing: mozilla/geckodriver#284 (comment)