Can't use numerics as custom suffixes when allowSuffixToValue is true

[src-code]

Right now it's not possible to have a custom suffix that's numeric if a rule has allowSuffixToValue set to true, such as Fz-10. Currently such a class would be translated into:

.Fz-10 {
  font-size: 10;
}

...which isn't valid.

The problem is that sometimes numerics are valid values, such as in the case of z-index, font-weight, or line-height, but in all other cases, a unit of measure (eg px) would be required.

I propose that we add a boolean to the rules that basically declares that for a given rule, the suffix-to-value requires a unit of measure. Something like suffixRequiresUnit, though I'm not sure how to best word it. By default this setting would be true, but would be false for the exceptions such as the ones listed above.

@renatoi @thierryk thoughts?

[thierryk]

Makes sense to me