Introduce support for audited_class setting per model

[amkisko]

Done:

• test coverage for all changes in this PR
• README updated accordingly
• generators updated to support custom table name

Background:

• single table might become problematic for large amount of logs/audits
• partitioning of audits table leads to other problems to solve
• having full separation of audits tables per model gives more flexibility
• keeping it strict and as much verbose as needed
• compatibility with previous behavior required
• looked at Define audit_class per model #641

[amkisko]

@danielmorrison Hey! These changes has been tested in production for past two weeks, no issues found so far, works quite well.

[amkisko]

@danielmorrison CI tests are all actually green, there is this "missing coverage" assessment, which does not return anything locally (meaning that all changes are covered), but here it returns different results depending on Rails version and database driver.

[amkisko]

Any suggestions how to replicate it locally?