Attachments in messages are ignored

[NikolayRys]

Hi! Current implementation does not have any insights in messages that contain attachments, like this:

It does not even see the starting piece Event starting in 5 minutes:, but it would be totally reasonable to expect to happen. Since actually it's being a title of an attachment, it causes this slack adapter to miss it. We had to develop such feature for an internal project and currently we maintain it as a monkey patch, which is very inconvenient. I mean, a feature, which provides ability to match attachment components with regular routes(here it would beroute(/Event starting in 5 minutes:/) do ; end) and access to the attachment in the route body.
So I would like to submit it here, as a proper feature(turned off in config by default) with adequate test coverage. If I do, will this be accepted? I guess people can use that.

[NikolayRys]

Ok, I'll do a PR still, maybe then it'll be easier to evaluate the idea.