Improvements to dotenv CLI

[cadwallion]

This replaces the odd file parsing in the Dotenv::CLI with a proper OptionParser implementation that automatically handles converting the comma-separated list into an Array and with proper flag support.

This also adds --version and --help flag support to the CLI, since it's practically free with OptionParser.

Completes #371
/cc @olleolleolle

octobox dotenv (help) ./bin/dotenv --help
Usage: dotenv [options]

    -f FILES                         List of env files to parse
    -h, --help                       Display help
    -v, --version                    Show version

[olleolleolle]

I like how readable the implementation of the options becomes by the introduction of the “controlling method” which calls the individual add_ option methods.

Having version number output helps operators.