Clean up copied & pasted code in HTMLFormatter and the spec

They were just copied & pasted from JSONFormatter and the HTML output
was actually not tested.
Use the same color scheme with other formatters

* Convention, Refactor - yellow
* Warning - mazenta
* Error, Fatal - red
Highlight code with markup rather than the primitive clang-style text
Markup code snippets in offense messages
Improve wording
Omit offense location information other than line number

Column number and length are obvious at a glance by the code highlight.