Providing more validation of string fields

[lumjjb]

From: #128

• Validation: There are a number of fields right now that aren't closely checked for validity. As just one example, I don't believe SPDX IDs are checked to confirm that they use only permitted characters; many of the fields are plain strings and are not checked for particular formatting; etc.
  • A validator should also take into account whether and how it is used in connection with a loader. For example, tvloader could be adjusted to take a parameter to optionally fail on parsing, if the parsed document is not strictly valid.