Added setCompleters(...) function

Added setCompleters() method, so that completers can be explicitly set from a pre-populated array, or can be removed entirely (if null is passed, or setCompleters() is called without an argument or an empty array is passed.

Please check this carefully for rookie JavaScript bugs, as I'm not sure if the null/undefined test is best-practise here as I don't typically code in JavaScript in my day-job.