Make http 429 and 206 codes return alive

[NicolasMassart]

Added features

• http 429 code retry option.
  The link-check library used to check links here have this option but it defaults to disabled. I added a CLI option (-r or --retry) and also the ability to add the option to the JSON config file.
• option to define the http codes for which an alive status is returned. I did not only make 206 work, but any code and it's configurable. Default is 200 only.
  CLI option and JSON config options are available. In JSON the list of codes has to be an integer array, on the command line a list with values separated by commas.

Testing

• test suite was updated to check for 206codes to be considered alive and for retry on 429 with a duration returned by headers.
• no CLI test are already present in the library so I did not add any. Maybe a TODO...

Doc

• CLI help updated
• README updated

Related issues

• Fixes my own issues
• fixes Add Throttling #101
• fixes Filter Based on Status Response #94

[NicolasMassart]

FYI I sent an email to the @tcort to know if he needs help on this project and if this can be reviewed.
If nothing happens next week I will merge in my own fork master.
@tcort is still here ;) Feature merged!

[NicolasMassart]

Thanks @tcort !

[NicolasMassart]

Update available for this PR in #123