Add custom assert message

[nelsonthedev]

Hi, it would be nice to have the ability to provide a custom assert error message on the visualDiff functions, so that it looks better in the reports.

Current assert error message:
MissMatch Percentage 1.86 + expected - actual -false +true

Desired assert error message example:
Images A and B do not match. (MissMatch Percentage 1.86) + expected - actual -false +true

[puneet0191]

Sure, go ahead and help with the Patch if you can :) otherwise I look into it in some days, thanks a lot