Let specify full-path argument for --browser

[neojski]

Due to issue https://github.com/substack/browser-launcher/issues/38 I have to add symlink /usr/bin/google-chrome to /usr/bin/google-chrome-stable because browser-run depends on the browser-launcher detection.

I would be useful if I just could say browser-run --browser /usr/bin/google-chrome-stable. Or just a different flag browser --browser-bin /usr/bin/google-chrome-stable

[juliangruber]

That's not straight forward to add with the browser-launcher dependency...maybe try coming up with a pull request for browser-launcher, then we can add this here