Extract mappings needs to also pull the 'Status' column from document_mapping.csv
Status needs to be passed through to the output CSV

This is an interim change that passes status all the way through but just adds it as an extra column, i.e. the logic to figure out whether it's a 418, 410 or 301 is not included in this commit.
Logic to determine whether status field in output CSV is 418, 410 or 301
Committing the tests used for propagating 418s

These are not fully formed tests but I don't think this is worth the time given that the whole munge process is about to be refactored. However, they were useful and took time to put together so committing, for probably later deletion...
Updating validation so that it does not fail when source csv contains 418 status

At the moment the 418 status is ignored by the mappings generation so it will not fail validation but these mappings will not be created.
Merge pull request #7 from alphagov/pass_418_status_through_munge

Pass 418 status through munge