Added feature for creating draft posts.

Default usage is:

$ jekyll draft "Some new draft post"

which will create the file ./_drafts/some-new-draft-post.markdown with some default front-matter.

Options incude:

* -f / --force                       overwrite an existing draft
* -t TYPE / --type TYPE              specify the extension, e.g. markdown, md, textile, etc
* -l LAYOUT / --layout LAYOUT        specify the layout, e.g. post, about, etc