Generate blog post easier

For example:

    rake gen_blog_post\[super duper announcement\]

This will create a file in
`blog/_posts/%Y-%m-%d-super-duper-announcement.md`.

If `VISUAL` or `EDITOR` is defined, the file is opened immediately for
editing.
Merge pull request #398 from BanzaiMan/generate_blog_post

Generate blog post easier