retain html report

[gravityvi]

Changes

• Add a flag to retain HTML report across test runs
• Default behaviour is not changed it generates different HTML reports if retain_report is set to true

Impacts

• fixes #3307

[gravityvi]

default output_folder would is now moved inside reporter_options however output_folder provided directly under config would also be considered.

If folder_format is provided Nightwatch would retain HTML reports across test runs. For example

// nightwatch.conf.js
{ 
 reporter_options: {
    output_folder: 'test_outputs',
    folder_format: () => Date.now()
 }
}