Small improvements

[triumphvalley]

Went through my markdown file and implemented some stuff:

• Improved native menu: More accelerators and "Reveal in Finder" for markdown file and template folder
• directly overwrite the global Reveal object, so users don't have to call RevealEditor.initialize etc.
• Also overwrite the configure function with our custom options.
• Introduce a variable MOONSLIDE_ENV, which is one of the values preview-small, preview-fullscreen, export so users can customize their entry script.
• Removed some stylings
• Directly referenced stylesheets inside config.yml
• Improved reloading of previews
• Close and repoen preview window when it is opened.

[triumphvalley]

Don't merge yet, still has some problems, I noticed