Add `unicode_strict` param to `filter` method

As it stands, the `filter` method will miss an item whose creators name is "Noël" when the query is "noel". Since I don't expect users to input Unicode formatted queries into Alfred, I added a parameter to the filter method that will normalize Unicode strings to ASCII strings. This allows the query "noel" to match on an item whose creator is "Noël". 
I've added documentation in your style and format. I've also added a new test to the test suite.