Move configfile parameter to help table

Moving the configfile parameter from the config table to the help table is the
first step toward the implementation of the config file generator.
Make the usage string use \z

The escape character \z lets us format the string while keeping the code clean
and indented.
Change configuration option names

Each option name is prefixed with the activity it is related to. This change
makes the options (a bit more) self describing and also makes it easy to sort
options alphabetically while keeping them logically grouped.

The .luatodo config file has been updated as well, so that applying changes
won't slow or even break the normal workflow.

The README file has been updated to reflect changes.

This commit represents another step toward the implementation of the config file
generator routine.
Cosmetic changes

A scoped auxiliary variabile has changed name in order to allow a source code
wrapping at 80 characters.
Fix typos in verbose messages
Add special interpretation for - in output_file

There may be situations where writing the recap to a file is not advisable or
even impossible.