Expand AlertNode message functionality

Hey guys,

I [posted](https://community.influxdata.com/t/using-kapacitor-vars-within-alertnode-message-property/2722) on the community forums little while back, but it's more of a feature request, so I thought I'd make an issue. Hopefully this isn't stepping on any toes.

Basically, I feel like it would be beneficial if we could include `var` values within the alert message, as well as provide more string manipulation. For example, I have a var `timeWindow` that I would like to include in my message (Eg. `The total error rate for the last {{ index .vars "timeWindow" }} minutes is: {{ index .Fields "error_rate" }}`). Additionally. things like splitting strings, converting to lowercase, etc. would all help towards creating useful messages.

While I know the Alert node does a lot, I can see how it would be difficult to transform it into a fully functional alerting system, but if I can just provide more succinct info to other systems (Eg. send a json blob of all the data I need to Sensu), I can process it there.

I don't think either of these are possible right now, but I'm open to any suggestions if anything similar is currently possible - Perhaps with a UDF? Otherwise I hope you can consider it for a future release.

Start by locating the AlertNode message-template implementation and reviewing how vars and Fields are currently handled. Read the linked community discussion and the issue's UDF suggestion; before implementation, clarify which string operations and message data must be supported, since no file, test, or acceptance criteria is named.

Written by the indexing model from the issue text.