WIP: MarkdownConverter using reveal.js

A MarkdownConverter implementation using reveal.js to generate
a HTML presentation.
Template for presentations and splitting of slides

Implement the splitting of the markdown text into multiple slides and also
use a separate HTML template file for the presentations.
Use data-separator attribute to split slides

Use the data-separator attribute feature of reveal.js to split the
markdown document into multiple slides.
Improve handling of ENABLE_HOEDOWN in option dialog

Don't not remove the general tab in the option dialog anymore. Instead
just remove the hoedown item from the combo box.
Use CDN for stylesheets
Add support for MathJax and Highlight.js plugins

Add support for MathJax and Highlight.js through Reveal.js plugins instead
of adding the scripts to the HTML header.
Add abstract Template class and implementations

Move the handling and rendering of the template to its own class. The
implementations are not yet complete.
Use https to fix problem with proxy preventing access here
Complete Template implementations and add support for "Export to HTML"
Add more tests to DiscountMarkdownConverterTest
Add unit test for RevealMarkdownConverter
RevealMarkdownConverter does not support changing any converter options
Change discount configuration on travis-ci

Fix travis ci build by adding --with-id-anchor to the build configuration
of the discount library.
Merge branch 'feature/presentation-creator' into develop