Allow empty body ?

Hi,

More a discussion than a bug, I'm wondering why preventing to extract frontmatter attribut from md file without a body ?

I made a minimalist PR (1 char) to allow empty body in #19.

Reproduce issue:
```py
import frontmatter

s="""
---
foo: bar
---
"""
assert frontmatter.Frontmatter.read(s) is not None, "shouldn't is be not None?"
```