Add support for browser binary setting.

[snadon]

This PR makes it possible to use another chromium-based browser instead of Google Chrome.

At the moment #capabilities is called often and we could avoid merging the settings every time start_session is called.

I'm opening this PR to see how we would like to handle this properly.

[paulcsmith]

This is very cool and I think this is a fine way to handle it. Have you tried this out locally. This would be difficult to run automated tests on so before merging I'd like to confirm you've tested it and it works as expected

[redo-studios]

Yes I’m using it locally since yesterday. I tested with Brave. I did not try with other chromium-based browsers though.

[paulcsmith]

Awesome! I think that's great! Can I ask why you used Brave? I've never used it for testing before. Is it faster/nicer?