add --json option and associated logic

closes #6 (closed)

tests/fslstats/feedsRun

@@ -91,7 +91,7 @@ tests_with_preoptions = [
         "preoptions": "-K",
         "mask": "mask.nii.gz",
         "options": "-m",
-        "expected": "1.078044 \n1.028827 \n0.986428 \n1.020869",
+        "expected": "0.539022 \n0.514414 \n0.493214 \n0.510435",
         "name": "preopIndexMaskMean",
     },
     {