Add Ace template engine support

[tatsushid]

This feature is discussed at forum topic "Amber templates not working… documentation?". The needed upstream (Ace) modification was merged so it's time to send this.

This detects .ace suffix of a template file and treat it as a template written in Ace. It has a small trick

name = name[:len(name)-len(ext)] + ".html"

to avoid a layout call error (like "Unable to locate layout") at rendering pages because .html suffix is hard coded in many caller functions. It also has unexpected feature using multi template engine at the same time.

It only affects an internal template name, not a real file name but I think it's better to use a template engine independent name in caller functions or allow to specify a suffix it in future version.

[spf13]

@tatsushid Thanks for this great contribution. We need to add some brief documentation so people can benefit from this.

I've also been considering changing the extensions for all the templates. The default would be handled like the .html files now as a go template.

.ace.html
.go.html
.amber.html
...

What do you think of that?

Initial questions (helpful for documentation):

1. Can you interchange .html and .ace files?
2. Can an ace template be used as a partial?
3. Can an ace template be used as a view?
4. Can an ace template be used to override a theme file?
5. Where can I learn more about Ace & it's syntax?
6. Can I use the hugo template functions from within Ace?

[tatsushid]

First, I answer the questions

Can you interchange .html and .ace files?

Yes, I rewrote my site's template files one by one and replaced original ones by them for testing and got the same looks (not fully compared generated HTML codes). I used all of Hugo's template functionality except a short code in them. I'm ready for translation, for example, example/blog of Hugo by adding theme directory or Hyde for Hugo theme. What do you think?

Can an ace template be used as a partial?

Yes. Both in go format

{{ template "theme/partials/header.html" . }}

and in Ace format

= include theme/partials/header.html .

are OK. Please be careful the INTERNAL template name (template.New(name) name) must be specified as those argument, not the REAL template file name on the file system so it has .html extension instead of .ace. I think it is difficult to understand and should be improved.

Can an ace template be used as a view?

Yes. It can be used as same as the go template like

{{ .Render "li" }}

Can an ace template be used to override a theme file?

Yes. If it is placed in layouts directory, it overwrites the theme's file

Where can I learn more about Ace & it's syntax?

There are documents and examples

Can I use the hugo template functions from within Ace?

Yes, for example, the following Ace code with first Hugo template function

div.container.content
  div.posts
    {{ range first 5 .Data.Pages }}
      {{ .Render "content" }}
    {{ end }}

works.

On changing template file's extensions, I agree with you that it makes it easier to distinguish between HTML file itself and template file which should be converted in HTML. It also allows us to extend the extension for other types, for example, we can use

.ace.md
.amber.md
.go.md
...

if we add Markdown content generating functionality in the future.

[tatsushid]

I rebased the PR branch onto the current tip

[spf13]

Thanks @tatsushid .. Merged as a0d956c

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