Support a summary at the top of the report

This PR fixes #8 and is a prerequisite for implementing https://github.com/cucumber/cucumber-jvm/issues/292 so the HTMLFormatter in Cucumber-JVM actually inserts the summary data in the HTML report. The calculation of the summary data is implemented in Cucumber-JVM by https://github.com/cucumber/cucumber-jvm/pull/536.

HTML + CCS + JS are a little bit out of my comfort zone, but this seems to me as something like "the simplest thing that could possible work".