Fix Issue #56: Can't install latest version of npm globally

This works in tandom with a change in asdf core that will look for the
exec-path command and use it appropriately when found to conditionally
route the npm & npx commands to the correct executable path.

bin/exec-path

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+install_path=$1
+cmd=$2
+executable_path=$3
+
+if [[ "$cmd" == "npm" ]] || [[ "$cmd" == "npx" ]]; then
+  npm_bin_cmd=".npm/bin/$cmd"
+
+  if [[ -f "$install_path/$npm_bin_cmd" ]]; then
+    executable_path="$npm_bin_cmd"
+  fi
+fi
+
+echo "$executable_path"