Allow a user to specify a channel and username for the slack-webhook

In slack you can specify the username and/or channel to post to instead of the webhook-default. By using this options you allow a slack-user to use a single webhook for in multiple projects and still let each of them post to a separate channel.
Line is too long. [92/80]<br>Prefer single-quoted strings when you don't need string interpolation or special symbols.