Alternate Date/Timestamp Parsing Options

Thanks again for a great library.

I noticed when using your convenient `convert_dates` method, it only allows parsing of `%Y-%m-%d` style timestamps. My use case included hour and minute, so I extended the method to take in different time parsing, according to https://github.com/mbostock/d3/wiki/Time-Formatting. This third parameter is optional, so this is a non-breaking change for existing implementations.