Make data dir configurable

Having the data bundled with the code is very useful. However if I'm testing new data, e.g. to include boundaries for other countries, use an alternate set of data (e.g. historical boundaries), or want to use data that has a clear(er) license, then its handy to be able to override which directory the data is read from.

This small change allows the Geocoder to be constructed with a directory and adds a command-line parameter.