Save output parameters in json file

The main changes included in this release are:

• 1. The file with the parameters passed to post_fmriprep.py is now a .json file rather than .txt.
• 2. The .json file is saved in the same directory as the fmriprepr .tsv file is stored.
• 3. The argument previously called --scrubbing is now called --calculate-scrubbing-mask. This new name better reflects what the function actually does. If we pass this argument, the internally we determine which volumes should be scrubbed, in addition to some scrubbing stats, BUT we do not remove the bad volumes from the data. This change is reflected in the documentation at the top of the code.
• 4. Correct handling of the argument --confound_list.

If the user wants to remove confounds which are not the ones specified as default (["csf", "white_matter", "trans_x", "trans_y", "trans_z", "rot_x", "rot_y", "rot_z"]), then the correct way to pass the new list of confounds is:

--confound_list "confound1,confound2,confound3" --nconf 3

note that we need to specify the number of confounds via different argument (--nconf). The reason to do this is to make sure that the user really really wants to pass a different number of confounds.
This case is now documented at the top of post_fmriprep.py as CASE 5.