Feature Request: Make 'publish' topic handler an AlertNode chain method

Say that I want to add a logging topic handler to every one of my Kapacitor alerts, but those alerts already have alternate topics defined (e.g. for email alerts, etc.).

The current paradigm seems to be that I would set up ``publish`` topic handlers for each pair of topics. So if I had three topics A, B, and C, and my new logging topic D, in total I would need to create:

* A
* B
* C
* D
* publish A, D
* publish B, D
* publish C, D

I would then need to update the tasks using A, B, and C and migrate them to the ``publish`` handlers. This is kind of tedious.

What would be a much better user experience is if the ``publish`` handler were a first-class ``AlertNode`` chain method, so that all I had to do was add my new topic handler D, and then update each of the TICKscripts.

E.g.

```
...etc... | alert()
.publish(['topic1', 'topic2', 'topic3'])
```

Or alternatively...

```
...etc... | alert()
.topic('topic1').topic('topic2').topic('topic3')
```

Start by tracing the AlertNode chain methods and the existing publish topic handler in the Kapacitor codebase; the issue names no specific files or tests. Compare the proposed publish and topic chaining forms, then verify that a single added topic can be combined with existing alert topics without requiring duplicate publish handlers or task migrations.

Written by the indexing model from the issue text.