Subscribe does not enforce argument order

[Kuixz]

Refer #27

Not sure if this is a bad thing, so I write here.

In its current state the subscribe command in pkg/slack/command.go accepts filters before or after conclusions (e.g. /gha subscribe owner/repo branches:branch1 conclusion1 workflow:workflow1:conclusion2).

Stopping this behaviour is very easy. But I think more flexibility is better. What do you think @cychiuae