Support html files

[fuzzy76]

Would you consider supporting html in addition to markdown? This would make it easy to convert (or scrape) existing documentation sites for publishing using docbase.

[siddharthlatest]

@fuzzy76 html files are slightly trickier as they can be loading custom stylesheets, images from relative paths, other files that we would need to support as a result.

We want to support them on an as is basis, so it's easy to embed other documentation files within a project.