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??...
```
This won't do what you want it to do. `("yaml" or "yaml-md")` will be computed first and evaluate to `"yaml"` (since strings are Truth-y). The correct way is to do `if format in ('yaml', 'yaml-md')`. That said, I think we should get rid of `yaml-md`. It's technically more correct, however nobody that knows of yaml support will type in `yaml-md`.