Comments: Allow comment types to register a display callback

[adamsilverstein]

Add a render_callback arg to register_comment_type() that Walker_Comment
dispatches to when rendering a comment of that type. An explicit callback
passed to wp_list_comments() still takes precedence; built-in types set none,
so output is unchanged.

Rebuilt from #53 as
part of a stacked-PR trial.

Trac ticket: https://core.trac.wordpress.org/ticket/35214

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