Multiple alert message config

[jose-lp]

Description

Hello, I am using eslastalert in my company, I noticed that in each .yml recipe of a rule, only one message body can be defined. But in the company we need slack and email messages to have different information. I added this feature in this pull request.

Checklist

• [x] I have reviewed the contributing guidelines.
• [ ] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [x] I have updated the documentation.
• [ ] I have updated the changelog.

Questions or Comments

If you think it's worth it I can do a unit test and add the feachure to the other alerts to complete the pull request requirements.

This new feature allows both general and specific settings to coexist. If there is a specific configuration for an alert, the general one will be omitted. For example:

alert_text: hello
alert_text_email: email

alert:
-  email
-  slack

In this case the slack alert will use the general message and the email alert the specific one.

[jertel]

Did you see this in the documentation, under the alerts section?

Did you try nesting the email-specific settings under the email alert key?

[jose-lp]

That's exactly what you need, I overlooked it. Thank you very much, your application has helped a lot