New Rule: Octal

[CSenshi]

Rule Name: Octal

Rule Description: The field under validation must be a octal number

Rule Usage Example:

reqs = {"data" : "73021"}
rule = {"data" : "octal"}
validate(reqs, rule) # True

reqs = {"data" : "0o73021"}
rule = {"data" : "octal"}
validate(reqs, rule) # True

reqs = {"data" : "23489"}
rule = {"data" : "octal"}
validate(reqs, rule) # False, It fails because "8" and "9" are not in octal

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Before contributing please review RULES.md (check for duplication), also check CONTRIBUTING.md for more details :100: