File type sensitive README processor

Currently, it recognizes org-mode and plain text besides markdown.
To update only readmes, run
    shly update-project-readme-database
.

TODO
* rst(reStructuredText) : I know there is a command named `rst2html` but I couldn't find the executable.
* better plain text process : To show 'as it is', it uses `<pre>`, which is rendered with black background. Maybe it is stylesheet to be changed.