Provide a single source of truth for parameters

[Silverbullet069]

Right now, the parameters are mentioned in 3 different places: parse_params(), script_usage() and SCRIPT_FLAGS.

I want to reduce it down to two: the script_usage() and parse_params(). Or even better, one: parse_params().

[Silverbullet069]

It's not done yet, #13 is "double source of truth".