Customize alert message in topic handler

Just like alert filtering using the *match* directive, it would be nice if kapacitor would allow for custom message formatting to benefit from various alert target specific formats.
When sending to slack URLs can be embedded like `` while this is not suited for other alert handlers.
See this community discussion for more details:
https://community.influxdata.com/t/handler-specific-message-formatting/4722

Start with the topic handler and read the linked community discussion for the proposed handler-specific message formatting. Define how custom messages should be selected and formatted for different alert targets, then verify that Slack-style links can coexist with formats suitable for other handlers.

Written by the indexing model from the issue text.