commands, create: Add .Site to the archetype templates

This commit completes the "The Revival of the Archetypes!"

If `.Site` is used in the arcetype template, the site is built and added to the template context.

Note that this may be potentially time consuming for big sites.

A more complete example would then be for the section `newsletter` and the archetype file `archetypes/newsletter.md`:

```
---
title: "{{ replace .TranslationBaseName "-" " " | title }}"
date: {{ .Date }}
tags:
- x
categories:
- x
draft: true
---

<!--more-->

{{ range first 10 ( where .Site.RegularPages "Type" "cool" ) }}
* {{ .Title }}
{{ end }}
```

And then create a new post with:

```bash
hugo new newsletter/the-latest-cool.stuff.md
```

**Hot Tip:** If you set the `newContentEditor` configuration variable to an editor on your `PATH`, the newly created article will be opened.

The above _newsletter type archetype_ illustrates the possibilities: The full Hugo `.Site` and all of Hugo's template funcs can be used in the archetype file.

Fixes #1629

helpers/path.go

@@ -215,7 +215,7 @@ func (p *PathSpec) getThemeDirPath(path string) (string, error) {
 	}
 
 	themeDir := filepath.Join(p.GetThemeDir(), path)
-	if _, err := p.fs.Source.Stat(themeDir); os.IsNotExist(err) {
+	if _, err := p.Fs.Source.Stat(themeDir); os.IsNotExist(err) {
 		return "", fmt.Errorf("Unable to find %s directory for theme %s in %s", path, p.theme, themeDir)
 	}