Add repository stream context options

Add support for passing stream context options to the
StreamContextFactory. This allows support for SSH keyfiles, SSL
certificates and much more. Example:

{
    "repositories": [
        {
            "type": "composer",
            "url": "ssh2.sftp://host:22/path/to/packages.json",
            "options": {
                "ssh2": {
                    "username": "composer",
                    "pubkey_file": "/path/to/composer.key.pub",
                    "privkey_file": "/path/to/composer.key"
                }
            }
        }
    ]
}

src/Composer/Repository/ComposerRepository.php

@@ -27,6 +27,7 @@ use Composer\Util\RemoteFilesystem;
 class ComposerRepository extends ArrayRepository implements NotifiableRepositoryInterface, StreamableRepositoryInterface
 {
     protected $config;
+    protected $options;
     protected $url;
     protected $io;
     protected $cache;
@@ -46,7 +47,12 @@ class ComposerRepository extends ArrayRepository implements NotifiableRepository
             throw new \UnexpectedValueException('Invalid url given for Composer repository: '.$repoConfig['url']);
         }
 
+        if (!isset($repoConfig['options'])) {
+            $repoConfig['options'] = array();
+        }
+
         $this->config = $config;
+        $this->options = $repoConfig['options'];
         $this->url = $repoConfig['url'];
         $this->io = $io;
         $this->cache = new Cache($io, $config->get('home').'/cache/'.preg_replace('{[^a-z0-9.]}i', '-', $this->url));
@@ -199,7 +205,7 @@ class ComposerRepository extends ArrayRepository implements NotifiableRepository
                 $jsonUrl = $this->url . '/packages.json';
             }
 
-            $json = new JsonFile($jsonUrl, new RemoteFilesystem($this->io));
+            $json = new JsonFile($jsonUrl, new RemoteFilesystem($this->io, $this->options));
             $data = $json->read();
 
             if (!empty($data['notify'])) {

src/Composer/Util/RemoteFilesystem.php

@@ -30,15 +30,17 @@ class RemoteFilesystem
     private $result;
     private $progress;
     private $lastProgress;
+    private $options;
 
     /**
      * Constructor.
      *
      * @param IOInterface $io The IO instance
      */
-    public function __construct(IOInterface $io)
+    public function __construct(IOInterface $io, $options = array())
     {
         $this->io = $io;
+        $this->options = $options;
     }
 
     /**
@@ -241,6 +243,8 @@ class RemoteFilesystem
             $options['http']['header'] .= "Authorization: Basic $authStr\r\n";
         }
 
+        $options = array_merge_recursive($options, $this->options);
+
         return $options;
     }
 }