Provide configurable bot identity
Provide configurable bot identity

Further reading of the Slack API doc [1] indicates that :as_user is for
making a bot look like a user. This confuses me, but apparently passing
username and icon_{url|emoji} is expected for default bot messages.

This makes that identity configurable, but with OK defaults.

[1] https://api.slack.com/methods/chat.postMessage

markolson/linkbot