add enabling/disabling feature support

Okay, fixed that issue I was having sooner than I thought, so here is the refactor to make handling features pretty straightforward I think. @MoOx let me know what you think. Right now it doesn't smartly not re-parse based on whether it's looking for just the CSS or a plugin, but I think we could implement that by just using an extra option on `options` object, like `reparse: false` that's internal.
According to a recent update to rework-import, this is now useless. See https://github.com/segmentio/myth/commit/3078500d262811a53242985b915094a21367941a