feat: impl From<(r, g, b)> to CustomColor

[caojen]

Hi, this pull request implements From<(u8, u8, u8)> for CustomColor.

By this, we have one more way to create a ColoredString when using RGB:

"Hello".true_color(0, 255, 255);
"Hello".custom_color(CustomColor { 0, 255, 255 });

// new:
"Hello".custom_color((0, 255, 255)); 

[caojen]

@kurtlawrence Ok.

[kurtlawrence]

PR approved, I'll merge in once the CI runs through.