Overhaul the CLI parsing system

[grunt-lucas]

Refactor CLI parsing, it's a mess. We have a ticket here to refactor the project so there is a clearer client/library distinction, which would help this effort: #40

Look into the most well-supported C++ options parsing libraries. Porytiles currently uses the GNU getopt.h, which is certainly capable. But there may be more C++ friendly solutions out there that well help clean up the code.

Alternative Options

Take advantage of Python's C/C++ interop: https://docs.python-guide.org/scenarios/clibs/ We could keep most of the Porytiles codebase intact while also getting all the cool Python CLI handling stuff for free.

Alternatively, we could create Go frontend for the CLI and leverage the capable Cobra library. Would need to look into Go/C and Go/C++ interop.