Make display of incoming PM configurable

Currently PMs will show up as an incoming PM (like being whispered in games like World of Warcraft) but some people find this disconcerting.

Let's put that display behaviour behind a configuration (like we've done with the [timestamps](https://github.com/cabal-club/cabal-cli/blob/master/.cabal.yml-example)).

Start by reading the configuration example at .cabal.yml-example, especially the existing timestamps setting, and trace how that option is loaded and applied to incoming PM display. Confirm the current incoming-PM behavior, then verify that a configuration option can enable or disable it without changing other message display; no specific source file or test is named in the issue.

Written by the indexing model from the issue text.