Allow "all" to be used to apply to all available players on the server.

[svaningelgem]

Closes #163

[Krakenied]

The feature itself is really useful; however, I'm not sure if it's the best implementation that could have been done. The first problem with it is that the Bukkit#getOfflinePlayers method is such an expensive one, as already mentioned in the documentation. The performance impact wouldn't be that problematical if we used the method asynchronously and then synchronously operated on the data.

Also, I'm not sure if we shouldn't add some additional confirmations to commands ran with the all argument. It's easy to mistype the argument especially when a player has nickname starting with all prefix, which may lead to widespread data loss or undesired modification.

[svaningelgem]

I'm at a loss how to do that, but feel free to use the idea to build upon 😄

[LMBishop]

I shall convert this to a draft to be picked up in the future then