Add ability to use HTML tags in Markdown documents

The original Markdown specification by [John Gruber suggests using plain HTML](http://daringfireball.net/projects/markdown/syntax#html) for
tables and other advances things. GFM doesn't document the use of HTML but
it is supported and correctly rendered almost everywhere on GitHub (Issues, README.md, etc.)

Because of this, `markdown-preview` should support the use of HTML too,
otherwise it wouldn't be a Markdown preview tool.