Add description to ArgParser

It would be nice to be able to provide a description for the ArgParser, that gets printed above or below the `Usage: ...` line in the help message.

This shouldn't be hard to implement, so I could make a PR if you like this idea.

Start by locating ArgParser and the code that formats its help message, then inspect how the `Usage: ...` line is produced. The work is done when an ArgParser description can be supplied and is printed above or below that usage line, with coverage for the resulting help output.

Written by the indexing model from the issue text.