Add install script (#22)

* Add install script

* Fix linter issue

install.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# Based on Deno installer: Copyright 2019 the Deno authors. All rights reserved. MIT license.
+
+set -e
+
+case $(uname -sm) in
+"Darwin x86_64") target="Darwin_x86_64" ;;
+"Darwin arm64") target="Darwin_arm64" ;;
+"Linux aarch64") target="Linux_arm64" ;;
+*) target="Linux_i386" ;;
+esac
+
+version=${1}
+
+if [ $# -eq 0 ]; then
+    version="$(curl -s https://api.github.com/repos/thegreenwebfoundation/grid-intensity-go/releases/latest | grep tag_name | cut -d '"' -f 4)"
+fi
+
+version="$(echo "$version" | cut -d 'v' -f 2)"
+grid_intensity_uri="https://github.com/thegreenwebfoundation/grid-intensity-go/releases/download/v${version}/grid-intensity_${version}_${target}.tar.gz"
+
+bin_dir="/usr/local/bin"
+binary="grid-intensity"
+exe="$bin_dir/grid-intensity"
+
+curl --fail --location --progress-bar --output "$exe.tar.gz" "$grid_intensity_uri"
+tar xzf "$exe.tar.gz" -C $bin_dir $binary
+chmod +x "$exe"
+rm "$exe.tar.gz"