Support reading message from environment variable

[rawtaz]

One can currently set the SHOUTRRR_URL environment variable to specify the URL to use for sending a message.

Since both the URL and the message are the two main components of sending a message/notification, I think it makes a lot of sense to be able to specify the message using an environment variable as well. I also encountered that need right now, hence opening an issue for it. I suggest using SHOUTRRR_MESSAGE.

Is this something that you'd like a PR for? I can try to find time for one, otherwise if you want to do it yourself that's of course fine :)