Add a global setting 'misMatchPercentage' to define a KO test beyond thi...

Add a global setting 'misMatchPercentage' to define a KO test beyond this percentage (default is 0%).

A comparison of images may sometimes be slightly different (an offset of 1 pixel). In our case we want to identify a significant difference between two images and therefore put a test KO when the percentage difference is beyond a given threshold.