Add bracket syntax for addresses

[jcelerier]

In reference to ossia/score#128

Maybe this could be done by an attribute to the Destination class that would take a vector of ints ? (I did this in i-score)

If the vector is empty, it's the address, else we use the values in the vector to go from index to index.

e.g. something like :

Destination dest("dev:/address");
dest.setAccessors({13, 5, 17}); // will fetch dev:/address[13][5][17]

Else it could be another type, like TupleDestination.

[theod]

as I was working on mapping feature, I've added index feature for Destination.
does the following commit helps to solve this issue ?613b21f

[jcelerier]

Yep, I also added it into i-score now. Thanks !