Should be possible to diable all reports after being set with --reports

[Jyrki Lindroos]

There should be a way to disable all the reporting.

Normally, without using --reports, robocop will only output error messages
Using --reports All flag all the default reports can be enabled.

When using argument file where --reports All has been defined, it is not possible clear the reports from command line.

It would be good to have something like this:

robotcop -A arguments.cfg --reports None

to have only error lines outputted.