New HTML formatter

This is a HTML formatter. It has a simple "feature" div for Features and Scenarios, and no coloring. A future version may have a basic page structure option (<html><body>), in addition to the divs.

It is meant to be used with gollum, the github wiki (I can provide the corresponding css using gollum's default theme). The idea is to provide a gherkin wiki to Business Analysts, based on the same git repo than the developpers.

I am new to ruby, and this very first version is inspired from pretty_formatter, so I suppose you will want some changes before accepting it.