General Path Prefix for installer packages

Adds a new "installer-prefix" option to "extras", that allows to give a common prefix for installed packages. Prevents to restate the installation path for every new package added.

Use case:
- In your root folder, you create a composer.json and require a common web framework (such as cake-php)
- In addition to this framework, you require plugins for this framework

Now for example CakePHP gets installed to "vendor/cakephp/cakephp", but the plugins get installed to "Plugin/xyz" instead of "vender/cakephp/cakephp/Plugin".

Each and every plugin would now need to have their install path overriden with "vendor/cakephp/cakephp/xyz", which is error prone, because you always have to remember to not only include the packages into "require" but also into "installer-paths".

A global setting is IMHO the preferrable solution.

P.S: I'm preparing a pull request for a type specific installer-prefix.