Add toolkit (#64)

<!-- related issues, e.g. "will close #123" -->

## Description
<!-- describe in a few words the changes -->

Adds a simple toolkit module that sets a temporary NUPM_xxx environment
variables for development to avoid messing up your own setup.

toolkit.nu

@@ -0,0 +1,9 @@
+export-env {
+    $env.NUPM_HOME = ($env.FILE_PWD | path join _nupm_dev)
+    $env.NUPM_CACHE = ($env.FILE_PWD | path join _nupm_dev cache)
+    $env.NUPM_TEMP = ($env.FILE_PWD | path join _nupm_dev tmp)
+
+    print $'NUPM_HOME:  ($env.NUPM_HOME)'
+    print $'NUPM_CACHE: ($env.NUPM_CACHE)'
+    print $'NUPM_TEMP:  ($env.NUPM_TEMP)'
+}