Add support for parsing block_id and action_id interactive callback

[jeffreyjbrown]

Ran into issue when calling chat_unfurl Slack API method where the payload for unfurls: doesn't accept callback_id in the JSON. Resulting error: WARN: Slack::Web::Api::Errors::CannotParseAttachment: cannot_parse_attachment

From Slack support: Unfortunately, that[chat_unfurl] method doesn't support the use of callback_id Slack support informs me that we should instead be looking for block_id or action_id for the interactive callback for that method.

[jeffreyjbrown]

ah looks like issue #4 may solve this