[Feature Request] Scope based buttons

[ethernal]

Can you add ability to limit buttons visibility based on scope ex. text.md could then have buttons specific to markdown editing, but the same buttons are not required when editing cpp or other file types.
If undefined button should be visible in all scopes - that way you don't introduce BC and hard requirement on scope definition.

Re-posting this from: varemenos/atom-toolbar-almighty#24