Support for live reloading, Showdown upgrade, fixes etc.

- reload the present., keep the current slide on SPC
  => useful for previewing it while developing it
- moved the background div to the background
 not to prevent copying and clicking on the content
- style: brownish color for code to make it stand out
- readme improvements

- upgraded to the latest Showdown
- added info about Showdown extensions
- added links to related tools