[API] Add and expose isRunning

[ericlmao]

Exposing this variable on the API level will allow developers to run certain logic if the proxy server shuts down. To my knowledge, there is no way of telling if the proxy is shutting down besides the onDisable() method in Plugin, and that would only be invoked after clients are disconnected and whatnot.

[Janmm14]

During initial loading bungee plugins on bungee start just before enabling plugins, isRunning will also return false (i.e. static initalizer, constructor, onLoad).

This, as well as isRunning returning false during the shutdown process should be documented in javadoc.

[md-5]

Use case?

[Janmm14]

Use case?

Only use case I can see is inside PlayerDisconnectEvent (it should fire when bungee kicks players on shutdown I guess).