Expose more settings to command line interface

[okaneco]

• - Angle offset: radial offset defaults to 90º (π/2) so first color is at top of circle. 0º would start at 3 o'clock or due east of center.
• - Overlay factor: Blending mode for overlay is atop, and defaults to 0.9. 1.0 seemed to cover too much in one tone close to the center but would be good to expose to the user (almost done already).
• - Output file: currently always takes timestamp and never takes the user supplied output.