Adds additional linting to make description required (solves #264)

[miike]

This adds support for the description property and adds a new severity level for linting that makes the description property mandatory. Related: #264

[snowplowcla]

@miike has signed the Software Grant and Corporate Contributor License Agreement

[chuwy]

Looks good. Thanks @miike!

In the next version we'll probably change severity level notion to something more flexible (#232), so in the end this feature will have different API, but overall I like it!

[miike]

Agreed - that was something I was thinking about while implementing this. I wonder if it should be passed in via CLI or possibly have a minimal configuration file for the linter.

[chuwy]

I'm leaning towards CLI option. Something like --checks description,size,some_other_check and --checks all. I think scope of igluctl is too tiny to have a dedicated configuration format.

[miike]

Fair enough - a configuration file is probably overkill at this point.

[oguzhanunlu]

Cherry-picked to #309, closing.