AJAX Based Live Preview

Remove JS Markdown Extra dependency from post editor.

JS Markdown Extra still cannot handle markup pattern that comes with
HTML class and/or ID like this:

### Test Heading {.foo}

~~~ .html
Test code block.
~~~

![foo](bar.jpg) {.foo}

[foo](example.com) {.foo#bar}

I decided to use AJAX for post preview to make sure that the output will
be consistent start from now and in the future because here we use the
same parser that also used in the actual page output. This also save me
from doing complicated things like parsing shortcodes, that cannot be
done with JavaScript before.