Add interactive dialogs support

[rgomez90]

Mattermost now has interactive dialogs.

We should add support for it in the next release.

@PromoFaux should we maybe release a version 1.4.1 with support for interactive dialogs and updated README, since this is an easy task and leave v1.5 milestones as they are since they are bit more complicated/long.

I hope to be able to get all done in christmas.

[PromoFaux]

Happy to merge the readme changes, but I've requested a couple of changes.

Interactive dialogs can go into 1.5, and we can just push the other stuff back if we need to.

[PromoFaux]

This would be nice to have. Maybe I'll read the docs thoroughly and have a bash myself - but no promises. Removed from milestones for now