[Block] Explore showing context-specific crumbs and settings in the editor

[justintadlock]

Building off #14 and potentially #15, the block should do two things when there's a context in the editor (for example, when shown in the post editor):

• Show real breadcrumbs instead of placeholders where possible. For example, this might be Home > Category > Post Title for the post post type.
• Show context-specific controls. For example, when placed in the post editor, we limit the post_taxonomy controls to just the post type.

Research needs to be done on the various contexts that we can utilize, but I think a good starting point is just seeing whether this is feasible when placed in the post editor. Maybe even limit to Core post types as a start since those are more predictable. Anyway, let's explore.

Alternatively, if the real breadcrumbs don't work out, explore displaying the Crumb Type as a placeholder based on the user settings. We may even need this solution when in the Site Editor and not just as an alternative.