Console help refactoring

Fixes #4535
> Any suggestion about helper name and namespace?

Something like `yii\console\HelpParser`. No need `BaseHelpParser`.

> Instead of HelpSummary I'd use Summary.

I was using `helpSummary` because I want it to imply it is related to `help`. It also reduces the possibility of name conflict because `summary` and  `description` are very generic names.