Support justification strength

[carlgieringer]

• Conclusive: the justification conclusively achieves it's polarity on its target. It is sufficient to prove the polarity about the target.
• Contributes: the justification contributes to the polarity of its target, but it is insufficient on its own to prove the polarity about the target.

Duplication: different users may disagree about the strength of a justification. How do we account for this?

How does this relate to example-based justifications (#168)? I think example-based justifications are a sub-type of a contributory justification where the basis of the justification must be an example of the target.