Add User/offline player CommandElement to GenericArguments

[boformer]

Use case: Ban command, mail command, lastseen command.

[zml2008]

I can see this being useful. However, I'd like for User to be fully implemented before I expose a command argument type for it.

[MattBDev]

I think this can be added as soon as #580 is ready to be pulled.

[MattBDev]

Will this be in Revision 2.1 since issue #580 will add further support of User?

[boformer]

I guess. When the UserStorage is implemented, this will be easy.

[MattBDev]

@boformer I was hoping one of the Sponge team members could just add the milestone to this so it isn't forgotten

[MattBDev]

I see a few issues with this being implemented:

1. Being Case-Sensitive
2. Suggestions could be very performance intensive.

[boformer]

@MattBDev The methods for that are in the UserStorage service:

SpongeAPI/src/main/java/org/spongepowered/api/service/user/UserStorage.java

Line 126 in 99c57e8

Collection<GameProfile> match(String lastKnownName);

Adding the command elements is simple. The implementation of the service is harder.