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??...
```
Instead of redefining the variable below, it might be cleaner to do
```python
extension = "md" if format == "yaml" else format
make_filename =  = lambda entry: e.date.strftime("%Y-%m-%d_{0}.{1}".format(slugify(u(e.title)), extension))
```