Add a propType of any for ComboBox filter

Hi,

Loving the library so far.  I _think_ I've found a small bug in combobox.js

According to the docs I should be able to pass 
> Mixed - false, "startsWith", "endsWith", "contains", function(item)
into the filter property of ComboBox.

This seems to throw an invalid property error.  I was able to fix it by adding a filter propType of React.PropTypes.any.

I haven't included any tests for this but I'll try to follow up with some if you want to keep the pull open.