WTO: Store "Flag" attributes for "annually" category

Connected to #42 (closed) #24 (closed)

Blocked by #93 (closed)

• As a JSON repository
• I want to store the "Flag" attribute of the source files for the "annually" category
• in order to let the Web API consume it.

Acceptance tests

• A Flag column MUST exist in the TSV files corresponding to the original dataset.
• The Flag column MUST contain attribute codes (like E) or nothing.
• dataset.json MUST have a attributes_labels and attributes_labels properties.