Send extra arguments to addSource js function

[chandraiyengar]

I believe the extra arguments which are added on line 27 of sources.R are not being added to the addSource function call on the javascript side.

[chandraiyengar]

It would also be nice to have the possible extra arguments in the man page for the function, am happy to add these if you think so.

[walkerke]

Thanks! I'm fixing this separately but I appreciate your contribution.