Extend proof service to allow setting options for individual entailment

[ykazakov]

Currently all options for proof services are displayed in general preferences of Protege.
Sometimes it is useful to change settings while explaining a particular entailment, similar to the explanation workbench or the protege-justification-explanation plugin.
See Figure 1 and Section 4 of this paper for details.

This request came during the discussion of PR #3.