Plugin dependency configuration

[cheebo]

Plugin can request interfaces from the Registry.

Nori must support:

1. Registry can load multiple plugins with the same interface with no error.

For example, 2 plugins implement nori/Cache interface. One plugin implements this interface for reads, another plugin implements this interfaces for memcached.

It would be great to have opportunity to configure which plugin Registry must return on interface request by specific plugin.

For example, templates plugin requests nori/Cache interface and receives memcached but sessions plugin receives redis on the same interface request .

Config example:

plugins: 
  - nori/templates: # plugin id
    - nori/Cache: "nori/redis:2.1.3" # interface: id:version
  - nori/sessions: # plugin id
    - nori/Cache: "nori/memcached:1.0.2" # interface: id:version