feat: add the preview_styles parameter

[razonyang]

With the preview_styles parameter, you're able to use prebuilt or external styles.

params:
  decap_cms:
    preview_styles:
      - css/bar.css
      - css/foo.css

[yippyskippy]

@razonyang also clarifying this in your docs is a good idea. Does this relate to the the fix you implemented in #30 and thus registerPreviewStyle.

[razonyang]

also clarifying this in your docs is a good idea.

Agreed, will do if there is enough time.

Does this relate to the the fix you implemented in #30 and thus registerPreviewStyle.

This PR provides a handy way to registerPreviewStyle for prebuilt and external styles.

For SASS/SCSS preview styles, you can using assets/decap-cms/scss/_custom.scss (see #45) or the body-end hook for advanced usage.