Added stricter handling of cross-platform paths to the db module.

Instead of setting DEFAULT_DB to None, the module will now actively complain
if it cannot find a suitable location for the database.
Added a few configuration options to the web frontend.

The configuration file allows the port to be set, along with the
formatter used to render articles.
Moved configuration handling to the core of myweb.

This makes it easier for all frontends to be configurable.
Integrated the new configuration handling in all frontends.

 - The Tkinter frontend may now be styled via the configuration file
 - The web frontend was moved into its own subpackage
 - The web frontend now uses pluggable formatters, instead of hardcoded ones