Provide an API for validating URI Templates

Hi!

Would you consider an API to validate whether a particular input was valid under RFC 6570 (or does such a thing exist already and I've missed it)?

E.g., http://example.com/dictionary/{term:1}/{term is not a valid URI Template seemingly, but I cannot see an API that complains about that -- uritemplate.URITemplate will happily truncate the end part there and consider that a template with just one field.

(Even having URITemplate do enough validation of its inputs would also work).

Full context: JSON Schema Draft 6 adds a uri-template format. I'd love to use uritemplate to implement it in jsonschema.

URITemplateのエントリーポイントと、issueで説明されているRFC 6570の検証要件から始めます。検証APIまたは入力検証の動作を、malformedな例も含めて確認し、JSON Schema Draft 6のuri-template形式をサポートできることを確認します。