Force 'use' YAML value to be an array

Manipulated the parsed YAML front matter to force the 'use' value to always be an array even if it's passed as a single string. 

I feel like this really needs a test but I couldn't find a good place for it because Tests/FormatContextTest.php doesn't pass the data through the FormatManager. Feel free to suggest a better place to write a test if you want to include that before accepting the PR.