Seqlog

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* Support custom ``JSONEncoder`` implementations (tintoy/seqlog7 and tintoy/seqlog13).

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* Fix incorrect behaviour when configuring logging from a file (tintoy/seqlog10).
**Breaking change**: Configuring logging from file or dict will now by default override the default logger class to be ``StructuredLogger`` (this can be reverted to previous behaviour by passing ``use_structured_logger=False``).

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* Add PyYAML as a dependency (tintoy/seqlog6).

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* Improve documentation for logging configuration from file (3)

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* Implement and document logging configuration from file (3)

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* Fix sample code (2).