Add --artifacts option for generator.py

[ebeahan]

#873 included discussion for adding an --artifacts option to generator.py to specify which artifacts are generated in the script's output, and I agree this would be a useful addition.

Examples:

Current behavior with customizations: --artifacts csv,beats,intermediate
Exclude csv: --artifacts beats,intermediate

This feature won't consider handling asciidoc doc generation for custom artifacts as part of its available outputs.

[webmat]

Closing, as many artifacts require multiple files and subdirectories.

Folks should .gitignore what they don't need, and optionally move files from a temp output directory to wherever they want.