mark.channels is an object

[mbostock]

Broken out of #979. We’re already using a channel object for mark.initialize (since #801) so it makes sense to use an object here, too.

[mbostock]

Maybe the channels argument to the Mark constructor should be an object, too? We might want to add some accommodation for backwards-compatibility if we change that, though.

[mbostock]

Okay I did the above thing, and I feel good about it being more consistent. It does mean we’re slightly more lax (we won’t detect when a mark declares a channel that duplicates one already declared by the built-in styles), but I think that’s fine. I made this backwards-compatible just in case there’s a custom Mark out there. We might remove the backwards-compatibility eventually but this feels like the graceful approach.