Use console_scripts for CLI

[bollwyvl]

See:

• pip installable version of validator and scorer? UniversalDependencies/tools#147 (comment)

This PR moves bin/udapy into udapi/cli.py, and reworks the naming and structure to work as a main function rather than a "pure" script.

This is then referenced in setup.cfg to use console_scripts which will handle building cross-platform scripts at install time.