Enable custom scripting hooks for server events

[HosseyNJF]

It would be great if the shadowbox had the ability to hook custom executables or bash scripts on each event, pass environmental variables about the event, and even react to the script's exit code (inspired by OpenVPN's scripting). Some examples of events are:

• Client connection - variables: access key id, unique connection ID, source IP, etc. - On non-zero exit code: deny client connection (useful for accounting purposes)
• Client disconnection - variables: access key id, unique connection ID, source IP, octets used, etc.
• Server up
• Server down