Make `--colorful` default

[2JS]

Resolve #141

Make --colorful by default, introduce --no-color.

I think it's generally better to color by default, allowing explicit disable by --no-color.

Two concerns.

1. Leave or drop --colorful arg. Should I leave it for backward compatibility?
2. Naming --no-color. I think --no-color is great, but --nocolor, --monochrome, or any other is fine.

[2JS]

Updated to fix gpg signing.