Add CLI support for command templates/matrix commands #517

It would be useful to be able to generate the commands dynamically based on an environment variable. For example:

concurrently --matrix "A B C" "echo {}"

Which would be equivalent to typing:

concurrently "echo A" "echo B" "echo C"

Or with multiple values you could do:

concurrently --matrix "A B C" --matrix "1 2 3" "echo {1}{2}"

Which would be equivalent to writing:

concurrently "echo A1" "echo "A2" "echo A3" "echo B1" "echo B2" "echo B3" "echo C1" "echo C2" "echo C3"

This would be useful for common repetitive tasks that can happen in parallel. For example, let's say I want to download some docker image on a few different servers. I could do:

SERVERS="server1 server2"
IMAGES="python:3.11 node:20"

concurrently --group --matrix "$SERVERS" --matrix "$IMAGES" "ssh {1} 'docker pull {2}'"

This would help making concurrently a drop-in replacement for bash for-loops; the above would replace

for server in $SERVERS; do
    for image in $IMAGES; do
        ssh $server "docker pull $image"
    done
done

It would also make concurrently a drop-in for a lot of gnu parallel usages, eg:

parallel --quote ssh {1} "docker pull {2}" ::: $SERVERS ::: $IMAGES

Another example: processing a lot files in parallel, eg less compiling a number of files:

concurrently --matrix "$(ls *.less)" --group "lessc {}"