Improve file handling with lazy reads

[sirosen]

Primarily this changeset introduces lazy file reading to resolve #352. Additionally, it introduces a better caching mechanism to the schema file readers so that schemas read from ephemeral sources (like pipes) can properly be applied to multiple instances.

See also: pallets/click#2645

• Apply lazy file reading for instancefiles
• Fix schema readers to cache data
• Add a test which verifies lazy file reading