feat(comments): implement commentDepthLimit option

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.
docs(demo): use a comments depth-limit of 10

Reddit can have some deeply nested conversations sometimes, so I'll
cap it to 10 nested levels deep for now, although I think that might
be a bit much. It will be changed if it seems to be a UI issue...
Merge pull request #5 from caitp/issue-5

feat(comments): implement commentDepthLimit option