Support File Encoding validation

[insightindustry]

It would be good to add support for file encoding validation, specifically for two types of validation and checking:

• validators.encoding(value) which validates whether value is a valid file encoding type (e.g. string 'UTF-8'
• checkers.is_encoding(value) which checks against the file_encoding() validator

and

• validators.encoded(value, encoding) which validates whether value is encoded to the encoding charset
• checkers.is_encoded(value, encoding) which checks against the encoded() validator