Implement Configurable File Validation

[atonks2]

Because there are so many ICL specifications, not to mention vendors who deviate from those specs, this project needs to be more flexible in the way it validates files. One potential approach would be to implement a ValidateOpts struct containing flags to tailor the validation behavior.

For examples of how this pattern has been implemented in our other OSS repos, see:

• ACH: https://github.com/moov-io/ach/blob/719968e674cced90ace172b81e7ece9be8bca12c/file.go#L72
• Wire: https://github.com/moov-io/wire/blob/3b5b8d23fea6e4dbff6dd901ae3dd87096b95a64/fedWireMessage.go#L10