[Feature Request] --quiet flag for CLI

[MarcusNotheis]

Feature request

Is your feature request that we implement a new rule?
Please use the Rule Suggestion template instead

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
We have some projects where we have set the severity of various rules to warn. Running tslint combines issues for severity error and warn which makes it harder to find the issues that are really errors.

Describe the solution you'd like
A clear and concise description of what you want to happen.
I would like to have --quiet flag the the CLI which will change the output to only emit errors.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
I thought of using the output option and filter the generated file to only print errors, but I'm not that happy with that idea as we would have to manage this custom step in all projects.

Additional context
Add any other context or screenshots about the feature request here.
ESLint has implemented this feature: https://eslint.org/docs/user-guide/command-line-interface#handling-warnings

[dawncold]

should close this issue as the feature had been merged on #4025