Allow specification of `snakebids` config file

[pvandyken]

With the potential ballooning in size of snakebids apps CLIs, plus the difficulty of setting pybids filters via CLI, it would be useful to be able to specify a config file to be passed to the app (similar to fMRIPrep's --config-file. This is especially relevant for dockerized apps, where the shipped config cannot be directly modified with any ease.

Probably this can directly use --configfile from Snakebids, although I'd need to look into exactly how this would work. I'll post more details as they come.