chromium: Use Nix expressions for plugin settings.

We now create Nix expressions within the plugin output path(s) which
then will be imported and incorporated into the wrapper. This makes it
easier for other plugins to provide configuration settings to the main
Chromium wrapper.

Of course, in order to allow for external plugins we need to allow
passing a list of plugins to the Chromium derivation, but right now we
keep it internal and only use it for things such as NaCl (as soon as we
support it, of course).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>