include css in compiler output

[Rich-Harris]

Nice simple solution to #409. Eventually we'll need to get CSS sourcemaps working as well, but this is a good first step which should hopefully enable tooling like rollup-plugin-svelte to make server-rendering CSS a bit easier.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b9d3c23). Click here to learn what that means.
The diff coverage is 75%.

@@            Coverage Diff            @@
##             master     #417   +/-   ##
=========================================
  Coverage          ?   29.18%           
=========================================
  Files             ?       80           
  Lines             ?     2292           
  Branches          ?        0           
=========================================
  Hits              ?      669           
  Misses            ?     1623           
  Partials          ?        0

Impacted Files	Coverage Δ
src/generators/server-side-rendering/index.js	11.62% <0%> (ø)
src/generators/dom/index.js	57.14% <100%> (ø)
src/generators/Generator.js	40.75% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b9d3c23...b5d4b78. Read the comment docs.