Allow filtering instances before they become singletons

The DefaultSingletonService used to instantiate all the plugins matching
the given type, but one might want to narrow down what becomes a
singleton even further, e.g. when ImageJ2 is looking for the current
platform: it wants to instantiate all plugins implementing the Platform
class, but then only keep the one that claims that it matches the
current platform.

Therefore, let's introduce the filterInstances(List<PT>) method that
returns the unmodified list of instances by default, but can be
overridden in subclasses.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #17 from scijava/singleton-service

Allow filtering instances before they become singletons