Add support for default sub-commands

[jonahseguin]

This is necessary to allow for the following use case:
You have a base command (in my case, /factions)
You want the root command to display a list of available sub-commands
And you want each individual sub-command to have it's own class.
This allows you to specify the aliases within the @Command annotiation, instead of within the withSubHandler method (and still have the help description be built properly)

[jonahseguin]

Side note: i couldn't get your custom fork of intake to work in my maven dependencies for this intake-spigot project
so i had to change my dependency to com.sk89q.intake:intake - 4.7-SNAPSHOT
not sure how to fix this

I had to use this:

<dependency>
            <groupId>com.sk89q.intake</groupId>
            <artifactId>intake</artifactId>
            <version>4.2-SNAPSHOT</version>
        </dependency>

instead of this:

<dependency>
            <groupId>li.l1t.intake</groupId>
            <artifactId>intake</artifactId>
            <version>xxyy-82322fd</version>
        </dependency>

[literalplus]

The default sub-command part looks good, but please open a separate PR for the tab-completion stuff :)

[literalplus]

Prefer !isEmpty(), it's more semantic / easier to read in general