[minor feature] add visual cue for firejail outputs

[pirate486743186]

What i mean. When you start an app, firejail first spews some output, then immediately after that, the app spews it's own and they all look the same. Sometimes, these outputs are quite long, and it's hard to tell when one ends and the other starts.

It would be nice, if there was an obvious visual separation between the two. For example, every line starting with "firejail message:" . Or a long "---firejail-output---" at the beginning and end of firejail's output. Something of the sort....