Add option to enable or disable showing usernames in RSS items

Tweeper shows usernames by default in items created from multi-user
sites like Twitter or Instagram.

This is because  the main use case is to aggregate multiple feeds in the
same viewer, and in this scenario having some info about where the
messages is coming from can be useful.

However sometimes tweeper can be used to track one single feed and in
this case having always the same username repeated over and over is
unnecessary.

Make showing the username optional, but keep the current behavior as
default.

NOTE: for Twitter keep always showing the username in case of retweets
($screen-name != $user-name).