[Feature Request] Override severity

[marcos-diaz]

Please add a config setting as overrideSeverity=error|warning|null

Example use case:
John is collaborating in a project in which tslint.json defines defaultSeverity as error, but John does not want real errors to be mixed with lint issues in the diagnostics bar, so he would like to override all lint errors as warnings at editor level. With this option he could config Atom's linter with overrideSeverity=warning.