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
Explicitly import commands, adding shortcuts

To allow 'freezing' applications like PyInstaller 2.0 to work correctly
with the subcommands, it seems these commands should be imported
explicitly. As a bonus we can now easily add shortcuts for these
commands so that supplying only the first (few) character(s) of the
command is sufficient.