This reduces the need to pass vault_contents around in various places
and restricts Context to dealing with the actual note which is being
processed, instead of also carrying program state information.
This will help with future feature development as note parsing functions
can now access Exporter directly.
This refactors the Context to maintain a list of all the files which
have been processed so far in a chain of embeds. This information is
then used to print a more helpful error message to users of the CLI when
RecursionLimitExceeded is returned.
