allow operating sans transform

[yoshuawuyts]

There's no reason this couldn't work without a transform:

var css = require('sheetify')

var style = css`
  :host { color: black }
`

This can't however:

var css = require('sheetify')

css('tachyons')

We could like totes allow the former, and only require the transform for the latter. Benefits to this all: allow introducing sheetify gradually into workflows ✨ - tho using a transform would definitely be recommended for perf reasons (: