Implemented the no-clip mode.

Due to the SPIGOT-270 [1] bug, the inventory-based GUI for the no-clip state, implemented, cannot work. A command was added as a workaround (/spec b) to disable the no-clip mode.

[1] https://hub.spigotmc.org/jira/browse/SPIGOT-270

Closes #40.
Merge branch 'master' of github.com:pgmann/SpectatorPlus