Allow coexistance of undefined package types.

Hey!

I want to add following types: wordpress-language, wordpress-plugin-language and wordpress-theme-language. These require different style of installation as they need to be included as dropins. I coded my own plugin for this and really think it would be good way to go for these.

Some features in Wordpress  just require to be installed in certain way and multiple files need to coexist in same folder:
- muplugins
- dropins
- languages

There's some fixes on muplugins and with symlinks on dropins but those are really hacky type of solutions.

I have built a working prototype of dropin functionality in this plugin: http://github.com/koodimonn/composer/composer-dropin-installer 
and you can test it by using this composer file: https://gist.github.com/onnimonni/3a8ef68482803a1e7df8

People are using own custom codes every time for such things. Examples found on here:
http://discourse.roots.io/t/composer-mu-plugins/1326
http://discourse.roots.io/t/script-to-move-file-sunrise-php-to-app-after-composer-update/2140
https://github.com/roots/bedrock/issues/30

I would recommend to use one notation for these in 'extra' so that projects could be easily customisable but also easy to understand just by reading composer.json.

Cheers, thanks for composer!