Added username:n syntax and flag overrides.

username:n syntax:
This syntax adds a simple way to get the nth account to have a specific
username by using the format “username:n.” For example, to get the 3rd
account to have the username “Notch,” one could use Notch:3.

Flag overrides:
Used primarily in the `preview` command, this functionality allows
flags to override parameter keys. For example, the ‘w’ flag in /tr
preview is used for sharing the preview instead of making it span
multiple worlds (since previews cannot do that).