Fix whitespace and command-output handling of npm.cmd

[dklischies]

The current implementation of the npm wrapper on Windows does not work properly if the username contains a whitespace, because your AppData-path will then contain a whitespace.

Steps to reproduce:

1. Create a user account with a whitespace in it's name, such that your user directory path will be something like C:\Users\John Doe\
2. If you run, e.g., meteor npm version you end up with Windows' command not found error.

The same issue exists for the script path/first parameter. I encountered this when using npm-shrinkwrap: You will not trigger Windows' command not found error but module-not-found errors:

   error: couldn't install npm packages from npm-shrinkwrap: Command failed:
   module.js:340
   throw err;
   ^
   Error: Cannot find module 'C:\Users\Daniel'
   at Function.Module._resolveFilename (module.js:338:15)
   at Function.Module._load (module.js:280:25)
   at Function.Module.runMain (module.js:497:10)
   at startup (node.js:119:16)
   at node.js:945:3

I've also added an @-sign. This allows for easier parsing of the output of meteor npm version and the likes via scripts by suppressing CMDs behavior to always write commands in cmd-scripts to the command line. This way it is also consistent with the behavior of those commands on *NIX systems (which is why I would consider the current behavior a bug).

[dklischies]

Any feedback on this? Judging from #6679 I'm not the only one experiencing this. To clarify: This bug prevents every windows user from using meteor npm if his or her username contains a blank space. The fix is pretty straightforward. (Not sure who is responsible, maybe @benjamn?)