With the default config, there doesn't appear to be a way to add any new columns from the command-line. In particular, the only way to add columns from the command-line is with the --insert flag, which requires a defined Slot to use.

Two issues:

1. The default config has no Slots. It really should have at least one.
2. The --insert flag does not print any warning if there is no Slot. This means that, with default config, running e.g. procs -i Pgid just displays the normal procs output without informing me as to why it didn't work.

I'm also concerned that even if the default config adds a Slot column, I'll run into a case where I want to add more columns than there are Slots in the default config. This could be handled by allowing a single Slot to be replaced by multiple inserted columns (or by having a "Slots" that is used for any inserted column where there is no "Slot" available).