Timing middleware: add support for alternate message formatters

[cahna]

Hi, and thanks for the great library of utils! I needed support for structured log messages to avoid writing custom parser logic in my app's log aggregation service (Datadog). If you are interested in including such support, this PR is how I implemented that. Backwards-compatible changes:

• Add support for an optional message_formatter argument to add_timing_middleware()
• Test coverage for new feature
• Documentation for new feature
• Fix CONTRIBUTING.md to reference make docs-live

[julyvee]

This is exactly what I need, I hope it gets merged soon :) @dmontagu