Javascript hightlighting for NBConvert html output

Follow-up of #4448

Add a HTMLExporter.javascript_highlight CLI Bool to decide wether we use inbrowser highlighting or prehighlighted html.

Add a switch to nbconvert's markdown2html filter that will make pandoc output fenced code blocks as html \<pre\> blocks (no highlight) and rewrite the css code-classes so that google prettyprinter can pick them up and do the highlighting in the browser.

Add a html_escape filter. It is useful in templates that output raw code in \<pre\> tags. The code needs to be properly html-escaped because for example >= and <= will cause html parsing errors.

Update html_*.tpl templates accordingly to enable javascript highlighting.

The filters could not be part of another PR because they are necessary side-products of the feature being added.