Added support for using post categories in email newsletter templates.

@news 👏

@news I'm getting "Newsletter: Error running template, Error parsing template: template: template:53: function "in" not defined". If it's .Pages.Params.categories, is it an array?

@news it didn't work with in as it did for blog posts, but if I use range I can iterate over the categories and check if the one I need is there. Thanks!

@otaviocc I'm updating the help page about this. For each blog post in .Pages, there should be .Params.categories now. Let me know if that matches what you're seeing.

@manton exactly, as I mentioned I used range to access it. I noticed is that the same code from a blog post doesn’t work for newsletters. Take the following example which works for posts:

{{ if .Params.categories }}
    {{ if in .Params.categories "Article" }}
        🔗
    {{ else if in .Params.categories "Video" }}
        🎥
    {{ else }}
        🪴
    {{ end }}
{{ end }}

When the code above is used in newsletters, the compiler complains that the function in (doc) is not present. The solution, in newsletters, is to use range (doc) to iterate over all the categories for that particular post and check if the category matches the criterion.

{{ if .Params.categories }}
    {{ range .Params.categories }}
        {{ if eq . "Article" }}
            🔗
        {{ else if eq . "Video" }}
            🎥
        {{ else }}
            🪴
        {{ end }}
    {{ end }}
{{ end }}

Maybe newsletters are processed outside Hugo? And in is implemented by Hugo and not by Go?

@otaviocc Yes, newsletters are processed by a custom Go program I wrote, not Hugo. I wonder if “in” is implemented by Hugo. I’d love for this to be as consistent as possible. Thanks for testing!