Add the ability to have "optionally required" plugins

[adrienlauer]

In SeedStack, we would like to retrieve a plugin from another plugin only if it's present in the classpath and active. Like an "optional require". An "optional dependent" is something to be considered too, but we don't need this atm.

The kernel would provide the plugin in the InitContext only if it's enabled (or null if it's not).

[ejemba]

@adrienlauer Yes this is something we can add in the next version.

[pith]

I can work on this one, but do you have an idea about the API ?

I'm not sure that adding two additional methods optionallyRequiredPlugins() and optionallyDependentPlugins().

[ejemba]

@adrienlauer is that something still needed ?
I think the Optional java8 can help on this ?