This PR cleans up the `Command` classes in the CLI, introducing a
`DatabaseCommand` class for the commands operating on a database,
and a `getCommandLineParser` command to centralize the arguments
parsing and validation.
The opening of the database based on the CLI arguments and options
is now centralized in `DatabaseCommand.execute`, making it easy to
add new database opening features (like YubiKey support for the CLI).
Also a couple of bugs fixed:
* `Create` was still using `stdout` for some error messages.
* `Diceware` and `Generate` were not validating that the word count was an integer.
* `Diceware` was also using `stdout` for some error messages.
