dired-open: support for opening multiple files at once

[FrostyX]

In GUI file managers (e.g. nautilus, dolphin) it is common to select multiple files of the same type and open them at once. It is useful when for example opening multiple papers in pdf, playing a whole music album the old-school way, queueing multiple videos in vlc, etc.

I am suggesting implementing a function called dired-open-marked that would unsurprisingly open all marked files with dired-open preconfigured command for their filetype. It wouldn't call the external command n times for each file but only once and pass all files as cmd line arguments.

I would like to write such feature myself but first, I wanted some ack whether it is something we want to have in this repository (or at all if it is a bad idea).

What do you think?