Allow the security manager to be disabled with an configuration option

**Note**: I would consider this a temporary flag that should be removed after this feature has been given more consideration.

I do not think the impacts of this feature were thought out properly, although I agree with the intention of these. For example, DB pool implementations such as [BoneCP](http://jolbox.com) rely on the creation of threads. A related issue has to do with fetching via HTTPS (with HttpUrlConnection), which again spawns another thread.

When the property `net.md_5.bungee.disableSecurityManager` is set to `true`, a nag message is triggered and the proxy starts up after 10 seconds.