Go templates

`.tmpl` files use Go's `text/template` with `[[` and `]]` as delimiters, so they
don't clash with `{...}` in content and code. Output isn't auto-escaped.
Frontmatter works as in Markdown.

| Helper | Meaning |
| --- | --- |
| `[[ content ]]` | In a layout, the wrapped content; in a page, the body. |
| `[[ page "title" ]]` | A string value from page data. |
| `[[ partial "_partials/footer" ]]` | Includes a partial. |
| `safeHTML` | Returns its argument unchanged; kept for compatibility. |

The `page` helper only handles string values, and templates have no `pages`
helper, so lists like `tags`, `backlinks`, and listings are for JSX.