Fix file extension being appended twice. Fixes #4

Both migrator.rb and post.rb were appending ".html.markdown" to the filename.
Update specs to reflect duplicate extension fix
improves installation instructions
Merge pull request #7 from monfresh/fix-duplicate-extension

Fix file extension being appended twice. Fixes #4
Add newline between published: false and ---
Merge pull request #9 from monfresh/add-newline-after-published

Add newline between published: false and ---