select other columns in events.tsv as regressors

[jdkent]

Is your feature request related to a problem? Please describe.
If I want to treat errors as a separate regressor (as opposed to another trial type), then I will need to use another column of the events.tsv

Describe the solution you'd like
command line option to pass in other confounds from events.tsv

Describe alternatives you've considered
N/A

Additional context
N/A