Add SVG inline images, improve RGB color and length matching, fix bug

What's Changed

• css: improve RGB hex color and length matching by @hochhaus in #142
• css: add support for image/svg+xml for data-uri inline images
• html: fix double-escaping of content within HTML attributes
• tests: added more tests to provide examples of proofs of some open issues

New Contributors

• @hochhaus made their first contribution in #142

Full Changelog: v1.0.18...v1.0.19