Various grouping options for Observations

[vrom911]

Currently, we group Observations by filepath for the console output. However, it is not the only possible option. User can be interested in e.g. Severity grouping, or Category grouping as well.

We need to come up with a strategy to extend current settings.

[chshersh]

@vrom911 I think we will have an enum that defines all grouping options. Something like:

data GroupBy
    = ByModule
    | ByCategory
    | ByInspection

and then we will define separate functions that will perform appropriate grouping and header printing.