win,node-gyp: make delay-load hook optional

The delay-load hook that was landed in 3d46fef to make compiled addons
work on Windows regardless of the iojs.exe/node.exe filename causes
issues with a small amount of compiled addons.

Therefore this patch makes it an opt-in feature. An addon may set the
'win_delay_load_hook' option to 'true' in its binding.gyp to enable this
feature.

Example:

```
{
  'targets': [
    {
      'target_name': 'ernie',
      'win_delay_load_hook': 'true',
      ...
```

Refs: nodejs#1251
PR-URL: nodejs#1266
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>