Allow an options set to be a string as well as an object

[albertstill]

Hi. I wanted to be able to support more complex set option definitions at my company. For example:

{
  sets: {
    'font-h1': `
      font-size: var(--h1-mobile-font-size);
      line-height: var(--h1-mobile-line-height);

      @media (--tablet-min-width) {
        font-size: var(--h1-font-size);
        line-height: var(--h1-line-height);
      }
    `,
    'a-normalize': `
      color: inherit;
      text-decoration: inherit;

      &:hover,
      &:focus,
      &:visited,
      &:active {
        color: inherit;
        text-decoration: inherit;
      }
    `,
  }
}

I see this is supported in the consumers style sheet:

The code does a type check on the set and passes a string to Root#prepend instead.

[codecov-io]

Codecov Report

Merging #39 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #39   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          67     72    +5     
=====================================
+ Hits           67     72    +5

Impacted Files	Coverage Δ
src/visitor.js	100% <100%> (ø)	:arrow_up:

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[pascalduez]

postcss-apply@0.9.0