Can now generate output that is browsable offline.

Added a setting, "use-ugly-paths", which if true causes markdoc to
generate paths so that output files can be normally opened and
browsed in a web browser without being served by a webserver. Some of
the code from this commit on github:
https://github.com/bodylabs/markdoc/commit/a20a02cd020b035e2ded493549981de2ca0a1820

This setting is turned off by default.