initial release

This is a very rudimentary version that works for receiving the text from Slack Block Kit webhooks into Mattermost.

All it does is extract the text from blocks[*]['text']['text'] and concatenate them into a single message, with each block separated by a newline.

I will continue to improve this adapter if anyone has feedback. I'm especially interested to know what FOSS applications people are using that send webhooks in Block Kit format, so I can understand what message formats should be handled until Mattermost has official support.