Load rule ignores from external text file

[ssbarnea]

Ansible-lint will load skip rules from .ansible-lint-ignore file that is adjacent to its config file. The file format is very simple, containing the filename and the rule to be ignored. It also supports comments starting with #.

• Adds ability to load ignores from file
• Implements --generate-ignore for dumping ignores for current violations
• Adds documentation about ignores
• Change summary message to point to documentation for ignores
• This feature also deprecates progressive mode due to its performance and testing problems, like testing of pull-requests with multiple commits.

Fixes: #1584
Fixes: #3012
Closes: #2934
Related: #2880

[cidrblock]

Related: #3012

Dismissing my review now that ansible-test references have been removed.

[mattclay]

Now that ansible-test references have been removed, I'll leave the reviewing to the regular ansible-lint contributors.