Splitting multiple strings
I think you can use object literals. The user of your plugin can pass in an array of object literals: 

var settings = [
    {bg_x_speed_in: ..., bg_x_speed_out_set : ...}
    , {bg_x_speed_in: ..., bg_y_speed_out_set : ....} 
    , {bg_x_speed_in: ..., bg_x_speed_out_set : ....} 
]; 


In your code, loop through the array; each element contains the settings for one object.