Added --binary-arg option, which passes an argument to the browser binary.

[asajeffrey]

Added a --servo-arg command line option, and a matching servo_args parameter to ServoBrowser. This passes arbitrary command line arguments to Servo.

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[asajeffrey]

Handles #176, but just for Servo. Ideally there would be a generic solution for every browser, but I can't see how to do this without a breaking change.

[jgraham]

Hmm, what's the problem with making a solution that can work for all browsers (even if you don't implement it for all of them)?

[asajeffrey]

@jgraham depends how we do it. When I wrote that comment, I was thinking about adding an argument to *Brower::init, which is a breaking change. On thinking about it, we could just use the browser_kwargs function in the way that ServoBrowser does.

The only change this would make is for the option name to be --binary_arg rather than --servo-arg, and for it to be listed in the main options rather than the servo-only options (even if it is documented as servo-only).

It's a bit tricky for me to test on other browsers, not sure what we should do about this -- I don't want to break wptrunner for firefox, chrome, etc.

[asajeffrey]

I pushed a commit which renames --servo-arg to --binary-arg, so other browsers could use the same mechanism. @jgraham: does this seem better to you?