Show definitions at the top level instead of the first element that uses them

[dblanchette]

From a comment on #102:

Great tool! We have a very similar use case to above that would be solved by this. We have a number of common definitions and would like them to appear as top level type definitions rather than be embedded in the first element that uses them. For example: we have an address type. Would be great if this could appear at the root level and then these root level "types" appear in a table of contents style list.