Add support for setting CLI arguments using environment variables

[kehoecj]

Enable users to set all optional CLI arguments as environment variables instead of (or in additional to) directly passing them as CLI arguments. For example running export CFV_REPORTER="json would produce JSON output. If both a CLI argument and environment variable are provided the CLI argument should win over the environment var or any type of merging of the options.

Look into using the standard flag library but it may be a good excuse to refactor to COBRA since environment variable support is built-in

[kehoecj]

This can be closed when #112 gets worked unless we decide not to move forward with Cobra