Add method for removing tag contents

[mattico]

I'd like to be able to take an HTML snippet and remove everything that isn't plain text. This includes things like the contents of <script> and <style> tags. Essentially:

assert_eq!(ammonia::clean(r"
<script type='text/javascript'>XSS</script>
Some <em>text</em> here!
<style>a { font-size: 14px; }</style>
"), "Some text here!");

I'm willing to make a PR for this, but I don't know how the API should look. Builder::remove_content_tags()?

[notriddle]

Sounds good to me.