Improve reporting output

[reiterative]

The output of running doorstop with no parameters can include a lot of warnings and information that is not of immediate concern, making it difficult to see an error or a warning that requires action.

The --quiet option does not seem to suppress any of the warning messages.

It would be useful to have a tally of the different types of warning (suspect links, no links from child document, no links to parent document, duplicate level, unreviewed changes) and errors, which output at the very end of the report (after the generated tree)

If this is tricky to implement, it could perhaps be wrapped up as a 'doorstop report' option, which parses the logged output, accumulates the tallies and then outputs it all in a report file.