Arbitrary stack-of-stacks (maybe: `meta`/`deep`/`deref`)

[booniepepper]

Add an ability to have a stack-of-stacks (that is, a sigi stack of identifiers pointing to other stacks)

Needs some thoughtful design. It should be written in a way that allows for tree-like display

Naming could be something like meta or deref or deep?

[booniepepper]

Getting near to this with interactive mode. In whatever version comes after 3.2.2, this should be possible:

sigi -qt SOME_META_STACK list | sigi -

Where the items in SOME_META_STACK are sigi commands

[booniepepper]

It's more-or-less a thing now.