Add command to generate boilerplate argcfile.sh

[sigoden]

Use argc --argc-create <cmd...> to generate boilerplate file

For example, run argc --argc-create build test run will create argcfile.sh in current directory with following content

#!/usr/bin/env bash

set -e

# @cmd
build() {
    echo "build to be implemented"
}

# @cmd
test() {
    echo "test to be implemented"
}

# @cmd
run() {
    echo "run to be implemented"
}

eval $(argc --argc-eval "$0" "$@")