Support html previews

[AllanWang]

Relates to #9

Given that markdown can be converted to html, but not always vice versa, and that github exposes html bodies, it would be best to convert html to spans.

Android already supports this, though I'm curious about the performance and the backing framework, tagsoup.

Perhaps I will make something similar to codeview, using jsoup and coroutines

References

• https://github.com/slapperwan/gh4a/blob/master/app/src/main/java/com/gh4a/utils/HtmlUtils.java
• https://en.wikipedia.org/wiki/Comparison_of_HTML_parsers