Completion for commands

[The-Compiler]

Likely it's already possible to pass a completion function to a new command, but it should be documented.

[jgkamat]

toofar gave an example implementation of this in https://gitlab.com/jgkamat/jmatrix/merge_requests/1

[The-Compiler]

Something requested by @Earnestly: Somehow allowing the command to get further information from the completion; or allow the completion function to customize how to turn entries into arguments - to make something like this work: