The upside of this is that each command can define specific flags ; there is a
small loss of backwards-compatibility in that the command needs to be the first
argument.
`catala --help` will now only show a summary of commands, with more specific
manpages shown on `catala CMD --help`.
Another point is that the plugin interface is extended to allow plugins to be
registered as subcommands and have their own flags (this will be very useful for
adding flags to the lazy/dot/explanation plugin that has many options).
Note that no efforts has yet been made to specialise the options, the previous
type was just made global for all subcommands.
Should make it much easier and less error-prone to add new options. There is
still a bit of boiler-plate, but at least it's contained in the Cli.options
function and doesn't transpire in the interfaces.
