Attribute Constraints

This adds some attribute constraints. We validate certain constraints
based on information from http://www.graphviz.org/doc/info/attrs.html.
Validation errors are collected as part of builders, we still add the
attribute regardless or not if it fails validation, and build methods
return ValidationResult<T> where errors is a Vec<ValidationError>. We
still add attributes even if the validation fails so that users can
ignore validation errors and build appropriate struct.

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