add template functions to handle table output, fixes #176, replaces #296

[coryb]

This allows for templating natives to easily generate tables.

Specify headers like:

{{- headers "abc" "def" -}}

{{- row -}} will start a new row ready to accept cell data
and you populate the table cells with {{- cell value -}}, like:

{{- row -}}
{{- cell 123 -}}
{{- cell "ghi" -}}

So a full template would look like:

{{- headers "abc" "def" -}}
{{- row -}}
{{- cell 123 -}}
{{- cell "ghi" -}}
{{- row -}}
{{- cell "string" -}}
{{- cell now.Unix -}}

And generate something this:

+--------+------------+
|  abc   |    def     |
+--------+------------+
|    123 | ghi        |
| string | 1575327457 |
+--------+------------+

[mikepea]

LGTM!