Arguments/Configuration Refactor

In this PR I took the liberty to refactor how command line arguments are handled so that configuration for prospector can be specified from a .prospectorrc/setup.cfg/tox.ini file, PROSPECTOR_* environment variables, and/or command line arguments. Most of the heavy lifting in processing all these sources for configuration is handled by a library I've been working on called [setoptconf](https://github.com/jayclassless/setoptconf). This new rig should support all the same options prospector has had (unless I missed something), with the exception of --path. It now takes paths as positional arguments (but currently only processes the first path specified).

I've also tweaked the setup.py so that the prospector script is created/installed via a console_script entry point, rather than maintaining a bin/prospector script separately.