Read posts from file system, not manifest

Instead of having to manually list all posts in the `manifest.yaml`
file, they can easily be read from the file system. Post date
information can be stored in the filename instead. Version was bumped to
0.3.0 for this change.

Other more minor changes:

* Rename `manifest.yaml` to `config.yaml`.
* Move front matter separator into a constant.
* Replace reference to '.' path with call to `os.getcwd()`.
Add drafts folder for convenience