Rename id_manager to document_state.
Remove `document_root` member from `file_info`.

It isn't actually used any more.
Make member_action and friends available in actions.

A fairly clumsy mechansim for calling member functions in parsers.
Document the element flags and masks a bit better.

I'm considering how source_mode tags work for paragraphs, which touches
on the contextual block stuff, so I wanted to clarify it.

As always, contains some TODOs that might not ever be dealt with.
Use a slightly clearer name for the grammar's block mask.
Store source mode as tag rather than string.
Start 1.7
Don't disable the newline after block handler in 1.7.

It isn't needed, although it does mean that this might be surprising:
{{{
* xxx xxx xxx
xxx xxx xxx
[block]
yyy yyy yyy
}}}

which is now equivalent to:

{{{
* xxx xxx xxx
  xxx xxx xxx

  [block]

yyy yyy yyy
}}}