chromium: Allow config.chromium for PPAPI plugins.

This should make it easier to enable proprietary pepper API plugins
though nixpkgs config, so it can be easily installed using something
like:

nix-env -i chromium-stable

With something like:

{ chromium.enablePepperFlash = true; }

In ~/.nixpkgs/config.nix to enable pepper API based Flash and to avoid
the browser wrapper from Firefox entirely.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit c833d7ce162a3568c4f179e44fcbfe6bc2106348)