Changed how templates work, wrote up two doc pages

- Fixed a bug in the linkifier where links wouldn’t get converted if
there were more than one of them on a line
- The whole post->final html code is happening inside the template
module now, which needs to change eventually
- The code that parses a post into markdown is now optional (for
templates that want to be inline)
- Wrote up two documentation pages: the index/introduction page, and
the page explaining the basic Noddity format.