SessionExtension: added option handler to pass own SessionHandlerInterface

[milo]

• new feature
• BC break? no

Own session handler cannot be set via session.save_handler, the ini_set() accepts string type only.
Can be solved by decorator extension:

decorator:
    Nette\Http\Session:
        setup:
            - setHandler(...)

but propesed way seems more suitable to me.

[dg]

Thanks

[milo]

Thank you!