[pdal] --drivers and --options now available

[Howard Butler]

All,

I have implemented the frequently requested support for listing the --options and --drivers that are available for the PDAL command line utilities.

See https://github.com/PDAL/PDAL/issues/148 for an example. I'd be interested in hearing about how we should format this better, and what other driver-specific information we should try to pass down through this mechanism.

Thanks,

Howard