feat: add the decap-cms/hooks/body-end hook for customizing Decap CMS

[razonyang]

Replaces and closes #30

This PR adds a hook for customizing the Decap CMS, take registerPreviewStyle as an example.

To use this hook, you'll need to create the layouts/partials/decap-cms/hooks/body-end.html file on your site root.

// assets/main.scss
h1, h2, h3, h4, h5, h6 {
  color: red;
}

// layouts/partials/decap-cms/hooks/body-end.html
<script>
  {{- $css := resources.Get "main.scss" | toCSS }}
  CMS.registerPreviewStyle('{{ $css.RelPermalink }}');
</script>

If your theme using prebuilt styles (not using Hugo pipes), you can register it directly.

// layouts/partials/decap-cms/hooks/body-end.html
<script>
  CMS.registerPreviewStyle('{{ "/css/main.css" | relURL }}');
</script>

[yippyskippy]

Great! Thanks.