User togglable censoring (chat only)

[jmshal]

This, when enabled, would allow users to enable/disable the censor filter for themselves. I saw it requested on another censor/no-swear plugin thread on Spigot - and I agree it would be a very handy thing to allow.

Initially this feature will only work with chat, and maybe we'll support signs later on.

It would add a new command namespace /censor me (command) which would modify settings for the current user only. And maybe for admins, the ability to do /censor user (username) (command) to administer users settings manually (helpful in times where users are unable to follow basic command instructions 😄).

I'll try and get this done in time for 3.0.8, however it's likely to be pushed back to 3.0.9 if we run out of time for the next release. PRs are welcome!