[Fixes #1528] Default reference assemblies made extensible in DefaultAssemblyProvider

@pranavkm @yishaigalatzer 
Following Ryan's scenario, make `IsCandidate` assembly protected virtual as well. The user might want to include assemblies based on another criteria, rather than explicitly add stuff to the CandidateAssemblies.

e.g.
```
If (library.Name.EndsWith("Controllers"))
 ...
```