Return nil if meta_description or need_id are missing from a flow

Fetch method on the front matter model has been 
changed to allow an optional default to return if 
an attribute is missing instead of raising an error 
(same behaviour as the fetch method on ruby hashes)
Make Frontmatter.fetch a bit more understandable, and add more tests

Using splat arguments lets Frontmatter.fetch self describe more clearly.

Add a new test to spec what happens if any additional arguments are sent (as
*args will capture many arguments).

Add another test for some negative cases too
Merge pull request #77 from alphagov/catch_deploy_errors

Return nil if meta_description or need_id are missing from a flow