Add build tags to let users conditionally compile the plugin system

[tomkwok]

Some users, like me, may find directly modifying the source code a better approach to add small features / helpers as this avoids the of overhead of a Lua (or possibly JS #15) VM.

So maybe adding build tags to let users conditionally compile the plugin system (including the VM) would be a good idea.

[tomkwok]

Done: 59d5a31