Update custom commands in help after they changed

[jappeace]

This is my suggestion for solving #815

It works but hlint is upset. I'd like to solve that but I have a question first. At the moment two reactions are produced, one for refreshing the custom commands and another for whatever command is being changed. I'm not sure if this is desired at all.

If you do think it'd be good to have multiple reactions then I'd like to add an and combinator to the reaction API (because it already has an either, adding and seems fitting).

[rexim]

@jappeace do I understand correctly that this PR is ready for the review? If so I'll try to do that Thursday stream. Thanks for submitting!

[jappeace]

@rexim Yes it works. I'll join strim later ~18:00