Add onCallBacks so you can listen to a single callback

[TomSmith27]

This change will allow a client to listen to a single event callback, instead of having to pass in functions for all options on the Hub.

Also updated the README to reflect this,

And also added the default as Null instead of undefined for typescript as i think null is a better indication than undefined, and aligns with C# more

[TomSmith27]

Do you think we should merge the changes for the single callbacks in and raise an issue to address the register unregister for the the ICallback implementation.

I know the single onCallbacks work because i am using them in my project