Implement allow_empty field for formats

[Henrique A. Lavezzo]

Third-party files sometimes has lack of data integrity. It's not uncommon, you need to parse a date (like birthdate), and third-party file does not contain this information in the specified range.
Today, estratto raises error trying to parse the date format, 'cause ' ' isnt valid date to parse.

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Proposal change:

In formats node, we'll insert a new property called allow_empty receiving a boolean value. When this value was true, estratto will return nil value to avoid any problems, and keeping data integrity. (Using premise, if we don't get data from range, a nil data can describe the actual state)
If value is false or allow_empty omitted, estratto will raise errors as default behaviour.

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Technical usage proposal:

        - name: birthdate
          range: 137..144
          type: DateTime
          formats:
            allow_empty: true
            format: '%Y%m%d'

[Alberto Rocha]

allow_empty is a nice name!