Add blog.

Squashed from the following commit messages:

Add middleman-blog.
Update Gemfile.
Configure blog segment.
Add blog files.
Adjust layout options.
Add more details into layout.
Add container to body.
Extract header and footer.
Add blog post title.
Use relative assets.
Make links to blog and home usable.
Remove extra social icons in footer. Update links.
Enable directory indexes.
Fix relative links.