Slack request signing verification issue

[threesquared]

I was trying to use this library with slackapi/bolt however I get an error Cannot read property split of undefined which seems to be an issue with verifying the x-slack-signature headers in ExpressReceiver.ts

Are there any plans to add support for request signing verification?

[royletron]

@threesquared I had a go at this: https://github.com/royletron/slack-mock/tree/f/add-token-header. It needs some work, but got me started. I will get some tests and docs together and submit a PR when things aren't on 🔥

Assumes either a SLACK_SIGNING_SECRET env var is set, or that your signing secret is and empty string.