Display names are not always the same as usernames #20
Description
Occasionally, a user may have a handle @xyz in Slack but (e.g.) jarjar -u "@xyz" -m "test" will result in UserWarning: Response code 404: channel_not_found
This is because display names are not the same as usernames. As a workaround, you can specify a user's Member ID (found in their Slack profile).
You can also find their username using the first part of their e-mail address (e.g., if the listed e-mail address for @xyz is xyz_user@gmail.com, then their username is @xyz_user). At least, this worked the one time we ran into this issue...
Potential fix: If jarjar gets a 404 user not found error, try to look up the username attached to that display name (how?) and try again.