[FIX,TST] Fix math label in proof directive

[najuzilu]

This PR fixes the list index out of range error thrown when a proof:proof directive includes a math directive with a label (#2).

New test cases have been included:

• _proof_with_labeled_math: example of proof:proof directive which includes a labeled math directive
• _proof_with_labeled_math: simple example of proof:proof directive which includes a math directive (no label)
• _proof_with_argument_content: example of a proof:proof directive that require no arguments

In addition to that, nodes.emphasis() has been replaced with the CSS font-style.

[mmcky]

Nice work @najuzilu.

This is your project so feel free to merge when you feel ready. @AakashGfude is currently running some further tests now if you'd like any verification etc.

[AakashGfude]

Thanks @najuzilu , looking good.