Add global variable for defining use of all-custom color shades, in place of individual custom parameters for btn generator

[zvuc]

Petal currently uses global shade generator (.colorset()) in many places to utilize a shade of a specified base color to emphasize or differentiate from base color. The button generator (.generate-btn) mixin apparently was made before the global shade generator mixin, and it uses a separate 'custom' parameter to use custom-defined shade variables instead of using auto-generated shades.

In a generic use case, if one wants to use manually set color shades, it's likely that he will do so for all colors instead of just specific colors, so using a global variable like @disable-auto-shade-generator that controls the behavior globally would make more sense than having the 'custom' parameter in just button generator. (It simplifies the button generator mixin as well)

[zvuc]

Resolved with release of 0.9.0 and 0.9.1