Allow filter chains to be constructed with instances in the specification

[gsteel]

We should be able to create a filter chain with something like:

$chain = new FilterChain($pluginManager, [
    'filters' => [
        [ /** Spec Array */ ],
        new StringTrim(),
    ],
]);

This patch allows instances to given in the spec array and updates the documented types to suit

Relevant to:

laminas/laminas-inputfilter#149

[Ocramius]

Thanks @gsteel!