Update config.yml

Added fnirtConfig parameter for the registration workflow and the FSL FNIRT input parameter config_file.

See Nipype documentation for details:
http://nipy.sourceforge.net/nipype/interfaces/generated/nipype.interfaces.fsl.preprocess.html

FNIRT inputs, config_file:

"config_file: ('T1_2_MNI152_2mm' or 'FA_2_FMRIB58_1mm' or an existing file name)
        Name of config file specifying command line arguments"