Paragraphs and line breaking in HTML posts

Hi,

Thanks for Exitwp. I've used it for migrating my data from my (old) WordPress blog to Jekyll. ☺

My posts are in HTML, which is the default in WP. The migration ran smoothly, but I've noticed that Exitwp outputs the content of the posts as-is. Now, when manually editing HTML posts in WP one can omit `<p>`s and `<br />`s and they are automatically added using the [`wpautop`](http://codex.wordpress.org/Function_Reference/wpautop) PHP command; thus, all of the paragraphs and line breaks in Exitwp's output were not processed by the browser as such, since they were whitespaces.

The pull request fixes this. Using [this](https://gist.github.com/albertsun/1160201) Python implementation of `wpautop`. It changes the default target format from Markdown to HTML, because the conversion to Markdown loses data (I think you might want to add a warning about this in the comment on target/output formats in the configuration file).

Thanks,
Júda