Auto load update

If ```PDAL_DRIVER_PATH``` is not set, default to ```/usr/local/lib``` or ```./lib```. It will be assumed for now that Windows users will have ```PDAL_DRIVER_PATH``` set by OSGeo4W. GDAL takes the approach of installing/checking for plugins in a subdirectory, e.g., ```/usr/local/lib/pdalplugins```. We could consider doing the same.