Add author option to page in front matter

If no author is specified in the front matter, like:

    ---
    author: jesse
    ---

Or if the author name does not exist in _config.yml, like:

    ---
    author: name_that_doesnt_exist
    ---

The author will default to nclud. All of the author bios are managed
in /path/to/sketchbook/_config.yml
Merge pull request #31 from nclud/feature/authors

Add author option to page in front matter