build/css: Allow for Twig variable prefixes in content hashing

This updated regex will correctly match and update refs to:

    href="/css/styles.css"

and

    href="{{ prefix }}/css/styles.css"

and even

    href="{{ prefix }}{{ another.prefix }}/css/styles.css"

Issue #154.
build/js: Allow for Twig variable prefixes in content hashing

This updated regex will correctly match and update refs to:

    <script="/js/main.js"

and

    <script="{{ prefix }}/js/main.js"

and even

    <script="{{ prefix }}{{ another.prefix }}/js/main.js"

Issue #154.