Feature: Add target directory option:

 As expressed in Issue #4, it could be nice to configure the installation paths of the TYPO3 extensions and core. 

Example of composer file:

 {
  "name": "typo3-project",
  "authors": [
    {
      "name": "developer",
      "email": "developer@example.org"
    }
  ],
  "require": {
     "typo3/cms": "6.2.3",
     "typo3-ter/news": "*"

  },
  "extra": {
    "typo3-cms-extension-installer-path": "http/typo3conf/ext/",
    "typo3-cms-core-installer-path": "alternative_path/typo3_src"
  },
  "repositories": [
    {
      "type": "composer",
      "url": "http://composer.lightwerk.com/"
    }
  ]
}