Update language_tools.js

Added removeCompleters() function to language tools exports as it is not possible to access the completers array directly.

This is currently utilized by Ace GWT to cleardown existing completers (for entirely programmatic completers without snippets or  keyword completions) but I feel it belongs in the mainline (I have to manage a fork at the moment and there seems to be no other workaround).
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.
less java:)
Merge pull request #2169

allow removing completers