Allow welcome message to optionally be set by file contents

[patcon]

It would be rad if the welcome message could optionally be set from a file in the project root, like so:
https://github.com/edgi-govdata-archiving/archivers-greetingslack#about-this-fork

Why? Sometimes I set up bots like this for less technical organizations. Heroku (and editing envvars) is very advanced for them, but editing a file on github is something they are very comfortable with. In combination with auto-deploy, this allows them to more easily fend for themselves when I pass the project off to them :)