Shortcode support

Noticed that shortcodes are not rendered in the migration. I settled with using the RSS import.

I'd guess that most blogs use shortcodes if only for the captions around images. It would be impossible to support all shortcodes, especially custom ones, but maybe we could support the standard WordPress ones? I don't know what should become of all the WordPress specific classes they add though.

For example this:

``` html
[caption id="attachment_1457" align="aligncenter" width="1200"] My terminal [/caption]
```

would become this:

``` html

My terminal