Ignore platform packages

Here's my attempt at #1522.

If a package is present in the 'platform'  config option, that package will override whatever PlatformRepository finds.
The "1.0.0" value is a version and not a constraint, so you can't do `"ext-pdo":"*"`. I don't know if that's a good idea though.

```
"config":{
  "platform": {
    "ext-pdo": "1.0.0"
  }
}
```

It also assumes that only a single version of each platform package will be found.
Finally, no tests in there. I wouldn't know where to start...
I have tested it locally though with success.