Add postcss and terser filters

[valtlai]

This adds the postcss and terser filters to the corresponding plugins.

Using them:

{% set css %}
  body::after {
    content: "Hello, the CSS world!";
  }
{% endset %}

{% set js %}
  console.log("Hello, the JavaScript world!");
{% endset %}

<style>
  {{- css | postcss | safe -}}
</style>

<script type="module">
  {{- js | terser | safe -}}
</script>

@oscarotero What do you think?

[oscarotero]

Yes, I like it. Good idea.