Replace restricted confound sets with config files

[tsalo]

Closes #1252 and closes #1231.

Changes proposed in this pull request

• Design YAML-based configuration files that define confound sets for nuisance regression.
• Add new --derivatives argument that accepts paths to derivatives datasets.
  • The primary derivatives dataset (i.e., the one provided through a positional argument instead of the --derivatives keyword argument) is internally referred to as "preprocessing".
  • Derivatives are linked to the confound configuration files via keywords. E.g., --derivatives aroma=/path/to/fmripost-aroma.
• Remove --custom-confounds argument. This is a breaking change.
• Keep filtered motion parameters and original motion parameters in the motion file.
• Separate handling of the motion parameters used for outlier detection from motion parameters that are used in the nuisance regression.