Fix #100 (Improved escaping of spaced paths on Windows)

[noelleleigh]

Fixes: #100

I went through the npx operations on a Windows 10 user account named "name with spaces" in it to figure out where the issue arises. Since npm proper does work with accounts with spaces, I figured it was an npx issue.

The script crashes during the running of the child npm process to install the packages. Inspecting the arguments provided to the installPackages() function, I decided to try escaping the path in the prefix argument using the child.escapeArg() function. I left the asPath as false because setting it to true led to the script hanging for some reason.

With these changes, I can run npx where before I couldn't:

(Running in my npx local repository)
Before

C:\Users\name with spaces\dev\npx> node .\test\util\npx-bin.js cowsay hey
Error: EPERM: operation not permitted, mkdir 'C:\Users\name'
TypeError: Cannot read property 'get' of undefined
    at errorHandler (C:\Users\name with spaces\dev\npx\node_modules\npm\lib\utils\error-handler.js:213:18)
    at C:\Users\name with spaces\dev\npx\node_modules\npm\bin\npm-cli.js:83:20
    at cb (C:\Users\name with spaces\dev\npx\node_modules\npm\lib\npm.js:215:22)
    at C:\Users\name with spaces\dev\npx\node_modules\npm\lib\npm.js:253:24
    at C:\Users\name with spaces\dev\npx\node_modules\npm\lib\config\core.js:81:7
    at Array.forEach (<anonymous>)
    at C:\Users\name with spaces\dev\npx\node_modules\npm\lib\config\core.js:80:13
    at f (C:\Users\name with spaces\dev\npx\node_modules\npm\node_modules\once\once.js:25:25)
    at afterExtras (C:\Users\name with spaces\dev\npx\node_modules\npm\lib\config\core.js:178:20)
    at C:\Users\name with spaces\dev\npx\node_modules\npm\node_modules\mkdirp\index.js:47:53
C:\Users\name with spaces\dev\npx\node_modules\npm\lib\utils\error-handler.js:213
  if (npm.config.get('json')) {
                 ^

TypeError: Cannot read property 'get' of undefined
    at process.errorHandler (C:\Users\name with spaces\dev\npx\node_modules\npm\lib\utils\error-handler.js:213:18)
    at process.emit (events.js:159:13)
    at process._fatalException (bootstrap_node.js:387:26)
Install for cowsay@latest failed with code 7

After

C:\Users\name with spaces\dev\npx> node .\test\util\npx-bin.js cowsay hey
npx: installed 9 in 3.593s
 _____
< hey >
 -----
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

I included a unit test for installPackages() for this new behavior, modeled after the other previously-implemented tests.

[noelleleigh]

Whoops, didn't see #138 before submitting this. Brian's changes seem more elegant, so I'll close this.