Templating for HTTP Request Headers

Currently, one can have templating in the request body for the alert, but we cannot have a templated value in request header.

For example, to add a templated message in alert, one can do the following:
```
var alert = data
|alert()
.id('{{ index .Tags "host"}}/mem_used_percent')
.message('{{ .ID }}:{{ index .Fields "stat" }}')
```

**Our use case is to send team-name information as part of the request.**
The team name information is stored in influx and it will be available in the tags, as you can see below.

```
var data = stream
|from()
.measurement('mem')
.where(exclusion_list)
.groupBy('host', 'team')
```

Is there a way to templatize the headers like below:

```
alert
.log('/tmp/mem_alert_log.txt')
.post('http://my-service/some-url')
.header('Team-Name', '{{ index .Tags "team" }}')
```

Start by tracing the alert .header() and .post() entry points, then compare them with the existing templating path for request bodies. Done means a header value such as '{{ index .Tags "team" }}' is rendered from alert data and covered by tests for templated HTTP headers.

Written by the indexing model from the issue text.