Start of command separation rewrite
Add base class for commands in new 'common' module
Add dynamic import of command modules
Add first command: dispass add (draft)
Add 'gui' and 'help' commands
Change scope of some variables in 'CommandBase'
Handle command options/flags in CommandBase init
Allow custom ordering of options
Include Python version in version information
Add 'version' command
Drop args for -h, --help options for main command
Replace Dispass with DispassCommand

Use the `common.CommandBase` class for the main class to further DRY-up
argument handling.
Add some more docstrings to `common` members
Add method for passing flags between commands

In `common.CommandBase`, add `registerParentFlag` method for passing
parsed flags of a command to a subcommand.
Finish 'add' command so that it actually works
Add 'dispass list' command

Add dispass list command which is functionally identical to
the earlier command and arguments: 'dispass-label -l [--script]', but
with added '--dry-run' and '--silent' options.
Add 'dispass rm' command

Add dispass rm command which is functionally identical to
the earlier command and arguments: 'dispass-label -r', but with added
'--dry-run' and '--silent' options.
Rewrite 'help' command to use the new main class
Add options to usagestr of 'rm' and 'list' commands