Move toolkit environment setting to a command (#67)

## Description
Instead of setting the environment on module import, it is now done in a
command that needs to be called manually.

toolkit.nu

@@ -1,14 +1,18 @@
-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)
+export def --env set-nupm-env [] {
+    $env.NUPM_HOME =  ('./_nupm_dev' | path expand)
+    $env.NUPM_CACHE = ('./_nupm_dev/cache' | path expand)
+    $env.NUPM_TEMP =  ('./_nupm_dev/tmp' | path expand)
 
-    $env.PATH ++= [($env.FILE_PWD | path join _nupm_dev scripts)]
-    $env.NU_LIB_DIRS ++= [($env.FILE_PWD | path join _nupm_dev modules)]
+    $env.PATH ++= [('./_nupm_dev/scripts' | path expand)]
+    $env.NU_LIB_DIRS ++= [('./_nupm_dev/modules' | path expand)]
 
-    print $'NUPM_HOME:  ($env.NUPM_HOME)'
-    print $'NUPM_CACHE: ($env.NUPM_CACHE)'
-    print $'NUPM_TEMP:  ($env.NUPM_TEMP)'
-    print $'PATH: ($env.PATH)'
-    print $'NU_LIB_DIRS: ($env.NU_LIB_DIRS)'
+    print-nupm-env
+}
+
+export def print-nupm-env [] {
+    print $'NUPM_HOME:  ($env.NUPM_HOME?)'
+    print $'NUPM_CACHE: ($env.NUPM_CACHE?)'
+    print $'NUPM_TEMP:  ($env.NUPM_TEMP?)'
+    print $'PATH: ($env.PATH?)'
+    print $'NU_LIB_DIRS: ($env.NU_LIB_DIRS?)'
 }