feat(comments): implement commentDepthLimit option

Often websites like to limit the number of nested levels to display at a single moment, and allow additional comments to be retrieved separately.

There are at least two strategies to do this, one of which is to limit the amount of data which is actually returned to the web app at a time (and this helps to perform better, of course) -- But artificial limiting would be a good thing to do too, because it's much easier to demonstrate that in a demo (and because it might be useful to people who want an "easy" solution for sites which don't get much traffic anyways).

So, I'm proposing a few things... At the bare minimum, a configuration option is needed to support the max-depth thing, and of course the directives need to actually listen to it. Perhaps by truncating the model object itself, so that ngRepeat doesn't go crazy.

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The commentDepthLimit option (specified in commentsConfig via the depthLimit option, or as the comment-depth-limit attribute to the comments directive) allows the developer to issue an upper bound on the depth of nested comments to be rendered.

If the option is undefined, the default is to trim to a depth of 5.