added an option to specify initial values

I have added an option to specify 2 initial values, as Frank suggested. They default to NULL but if the user wants to, they can specify a 2 element vector and both elements have to be between the upper and lower bounds specified or an error is returned. 

Have a look and if you think this is a good idea, merge the changes. I realize this is a little late (since we also have to edit the help files); it took a little longer than I expected to write it and test and I was also trying to check the slopes but decided not to implement that.