Option an option to make an icon's colors overridable

[kizu]

For doing so we could replace the existing colors with currentColor, then move those colors to svgicon-… classes, so you could override them in CSS.

There are a lot of nuances, that could become sub-options, like should we replace black/unassigned colors with currentColor, what should we do when there are two colors (we could use currentColor and default fill from CSS this way), three (nothing? Replace only black with currentColor) etc.