refactoring html snippet from elisp to separate template file

I don't think embed html snippet in main org-page elisp file is a good
idea, instead, I think providing seperate html template would be a
better way. And, seperate, indenpendent html template make it possible
to provide more flexible theme and layout customization, and make the
elisp code look more concise.
Merge pull request #27 from xiaohanyu/feature/use-file-template

refactoring html snippet from elisp to separate template file