1.2.0 - Device shadow validation support

New features:

• Support for validation of device shadow requests before applying them.
• Last Will & Testament support.
• Ability to treat [] as a deletion request, if enabled (for interoperability with systems using that as the canonical delete).
• Automatic registration of file format handlers (just add files to the filefmt/ directory).
• Added PLAINTEXT file format.

Fixes:

• Various corner cases addressed.
• Device shadow is properly normalised after changes from different inputs.
• Top level changes in the device shadow now handled consistently.
• Suppress no-op deletion requests.
• Additional code structure improvements.

Full Changelog: 1.1.0...1.2.0