Support standard time formats in Kapacitor templating

i was trying to format my `{{.Time}}` template and wanted to use `{{.Time.format time.RFC3339}}` but i kept getting an error telling me that `time function was not defined`.

I was able to get it working using `{{ .Time.Format "2006-01-02T15:04:05.999999Z" }}`

https://golang.org/pkg/time/#Time.Format

Start at Kapacitor's templating entry point for expressions such as `.Time.format time.RFC3339` and the existing `.Time.Format "2006-01-02T15:04:05.999999Z"` form. Check the Go time package documentation for standard formats, then verify that a template using `time.RFC3339` renders without the current undefined-function error.

Written by the indexing model from the issue text.