Multi slides

[longavailable]

Made some changes to support multiple presentations in one branch:

• changed the default posts to slides collection;
• added a layout for slides collection and set it as default page;
• modified the previous reveal.html to make it work only for single file;
• merge the previous multi-files slide to one-file slide and add another;
• update the readme.md.

Note that, the only syntax change for [jekyll-revealjs] is that the separator for horizontal slides --- to <!--horizontal-->, vertical from -- to <!--vertical--> to avoid the conflicts with the front matter for specific slide file.

[dostoievsky]

Hey guys, why this didn't got merged?