shell_replace_with_output action

[Axlefublr]

I'm working on a new feature, and want to get feedback to see if I should keep it in my personal fork, or if it would be accepted to core.

There's a certain consistency in various mappable commands: you can either insert, append or replace.
More specifically, the shell_insert_output, shell_append_output, shell_pipe commands.

The issue here is that while you can prepend or append the output of a shell command, if you wanted to replace your selections with the output of a shell command, you would have to employ hacks:

1. press d and then prepend — this easily breaks, because in a lot of cases all of your cursors would merge into a single one, so you can no longer replace all of those selections, you only have one
2. use shell_pipe like ; my-actual-command — while this works, I think it's agreeable that it's a hack.
   First of all, most people probably don't know they could use ; like that.
   Second of all, the intention is different. If we want to replace selections with the output of a shell command, we never wanted to pipe to begin with, so it's just a workaround for something that should be doable directly.
   So, I intend to implement shell_replace_with_output (not sure about the _with there, looking for feedback on naming).
   It will replace selections, but won't actually pipe them.

[justinlovinger]

Along similar lines, I would like something like shell_pipe_insert and shell_pipe_append to insert or append output while piping selection.