Additional context

[amkisko]

Done:

• updated README with additional instructions
• implemented context storage and merging before audit record writing
• implemented test coverage for changed parts
• updated generators and templates

Background:

• comment:string is not enough for storing and accessing additional context
• known approach is to have separate column with additional serialized data
• would be nice to avoid context storage on auditable record
• would be nice to have global setter to add context once per request/job
• should be always optional to add more context

References:

• it will have minor conflicts with Introduce support for audited_class setting per model #735
• conflicts are resolved fully in https://github.com/amkisko/audited/tree/release

[amkisko]

This one requires improvement, I completely missed the point that audited_changes is stored as text. context should be same for better support.

[amkisko]

@danielmorrison fixes pushed here.