Export to Pelican

This allows you to export your JRNL to Pelican formatted Markdown. The exported files have the necessary metadata at the top, and then contain the body of the entry. Files are saved with `.md` extension.

These can then be used as source content to generate the files of a Pelican blog.

Example file:

```
Title: RAM?
Date: 2013-07-30 12:02:00
Tags: computer, RAM

8GB of RAM for my desktop??...
```
`pelican-md` was originally introduced in case someone later added a ReStructured Text export for Pelican (the front matter is a different format). In any case, I don't think that will be an issue/confusion with the option named `yaml`, so I've removed `yaml-md` as you're suggested (see c756770a671b7b2caf5e97834fa9a17236794dca).