Add new installer

From time to time, new users experience issues that stem from installing
the Python package in unforeseen environments. This results in poor user
experience. In order to solve this, this commit introduces a sh-based
installer that creates a script wrapper for running popper via Docker.

Since Docker is a requirement already, this has a higher likelihood of
working, once a Docker installation has been successful.

install.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env sh
+
+POPPER_VERSION="v2.6.0"
+
+OS_NAME="$(uname)"
+if [ "$OS_NAME" != "Linux" ] && [ "$OS_NAME" != "Darwin" ]; then
+  echo "Popper only runs on Linux or MacOS. For Windows, we recommend WSL2."
+  exit 1
+fi
+
+command -v docker >/dev/null 2>&1 || { echo >&2 "docker command not found. Aborting."; exit 1; }
+
+cat > ./popper << "EOF"
+#!/usr/bin/env sh
+docker run --rm -ti \
+  --volume /var/run/docker.sock:/var/run/docker.sock \
+  --volume $PWD:$PWD \
+  --workdir $PWD \
+  getpopper/popper:v2.6.0 $@
+EOF
+
+chmod +x "./popper"
+
+echo "\nInstalled version $POPPER_VERSION to executable file $PWD/popper\n"
+
+while true; do
+  read -p "Do you wish to move this binary to /usr/local/bin/? [Y/n] " yn < /dev/tty
+  case $yn in
+    [Yy]* ) echo "You might be asked for your (sudo) password."
+      if [ -d "/usr/local/bin" ]; then
+        sudo -p "password: " -- mv ./popper /usr/local/bin/
+      else
+        sudo -p "password: " -- mkdir -p /usr/local/bin/
+        sudo mv ./popper /usr/local/bin/
+      fi
+      echo "\nPopper is now available for all users in this system!"
+      break
+      ;;
+    [Nn]* ) echo "\nTo make the popper command globally available, add it"
+      echo "to a folder reachable by the PATH variable.\n"
+      break
+      ;;
+    * ) echo "\nPlease answer 'Y' or 'n'.\n"
+      ;;
+  esac
+done