Type specific prefix path

An extension to https://github.com/composer/installers/pull/62:

Instead of only providing a general prefix path for packages, the prefix path can now be tailored to installer types, via adding "installer-prefix-{type}" to "extras".

EDIT:
Just saw https://github.com/composer/installers/pull/52, which is conceptually similar to my pull requests. IMHO the approach here is simpler both on end-user and implementation side.

Thanks for having a look at it,

Chris