make tuple translation configurable

One can choose whether tuples are translated to js objects or arrays
depending on how it is used.
This can be interesting when using js libraries that encode tuples as
arrays. For example I use this for the DSL I am writing to implement couchdb design documents in Scala 
For example, see [this test](https://github.com/gnieh/sohva/blob/master/sohva-dsl/src/test/scala/gnieh/sohva/dsl/test/TestDesigns.scala#L98) where the js function returns a two element array (of different types). Using a tuple in Scala makes it easier to typecheck without the overhead of defining a specific class that translates to an array.