Added suggestionFilter. Filters out specific suggestions before the box is shown

I needed this function, because there were some circumstances where I needed to filter out some suggestions after the response came back.

I think the readme says it all:

* `suggestionFilter`: `function(suggestion) {}` called for every suggestion from the response, before the suggestions are shown. The return value of the callback should be `true` or `false`, whether the suggestion should be filtered out or not.

I also added a test for this option.