create: Use archetype template as-is as a Go template

This commit removes the fragile front matter decoding, and takes the provided archetype file as-is and processes it as a template.

This also means that we no longer will attempt to fill in default values for `title` and `date`.

The upside is that it is now easy to create these values in a dynamic way:

```toml
+++
title = {{ .BaseFileName | title }}
date = {{ .Date }}
draft = true
+++
```

You can currently use all of Hugo's template funcs, but the data context is currently very shallow:

* `.Type` gives the archetype kind provided
* `.Name` gives the target file name without extension.
* `.Path` gives the target file name
* `.Date` gives the current time as RFC3339 formatted string

The above  will probably be extended in #1629.

Fixes #452
Updates #1629

parser/frontmatter.go

@@ -13,6 +13,8 @@
 
 package parser
 
+// TODO(bep) archetype remove unused from this package.
+
 import (
 	"bytes"
 	"encoding/json"