Add support for SetAllowCodeGenerationFromStringsCallback

[pinepain]

SetAllowCodeGenerationFromStringsCallback() accepts

/**
 * Callback to check if code generation from strings is allowed. See
 * Context::AllowCodeGenerationFromStrings.
 */
typedef bool (*AllowCodeGenerationFromStringsCallback)(Local<Context> context,
Local<String> source);

callback as it argument.

[pinepain]

I'm moving away from PHP world and all my PHP projects going to be abandoned too. Abandoning this project too as I have no intent to continue working on it unless there would be strong request from community and commercial interest. No more updates or documentation will be made. If someone is interested, feels free to contact me using email specified in my GitHub profile.