Added access_token API override option...

[kyllerss]

I am not sure if this is the best way of going about it. I have a server-side FB login flow through which I obtain an access token. I found that the FB.api call the plugin makes is not aware of the session. Perhaps there is a better way of initializing the FB instance to be aware of the access token, but I could not find one.

I have provided some changes to the code to add an 'access_token' option that can be passed in manually.

If you know of a better way, I am all ears.