a possible way to let Github Pages use the result of the custom converter

As written on the document of Jekyll,
http://jekyllrb.com/docs/plugins/

Github Pages does not allow custom plugins to run,
thus we need to convert the site locally and push the generated files.

This is a possible way to accomplish it without changing the current structure,
but this requires some extra procedures.

We need first generate the site to ./htmls by this command
```
$ jekyll build --config _config.htmls.yml
```

Then, push the generated site to update Github Pages.
```
$ git add html
$ git commit -m "push the generated site"
```

I actually not sure about Github Pages interprets the :source option of _config.yml,
probably it does.

I reccomend to check the two commits one by one,
especially the latter one is a little heavy to load.
The first one is for new settings,
the latter one is to commit the generated htmls.

Note that I didn't include html/images to these commits
because it can be significantly heavy to review.