#2 Show preferred extensions

[mattsouth]

For the even cycle:
a b
b c
c d
d a

All arguments are shown as accepted under preferred credulous reasoning, however the real justifcation for this is that there are two preferred extensions {a, c} and {b, d}. It would be good to show these extensions.

Issues that this request include:
1. calculating the maximal admissible sets (current algrithm calculates the admissible sets around particular arguments)
2. Showing the extensions in the UI - perhaps with subscripts on the arguments or using some kind of drawn sets.