Is there any equivalent to bracket notation in templates?

I’m trying to pass a tag name into a shortcode and render n items. I haven’t found a way to list content under a certain tag when that tag is passed into a shortcode.

For example, this is the way you would do this in a template where you know the tag you’re trying to list:

{{ range first 5 .Site.Taxonomies.tags.alabama }}
  {{ .Page.Title }}
{{ end }}

But what if you’re passing the tag name into a shortcode?
What if the tag name contains spaces?

In my Markdown:

{{< content-by-tag tag="alabama" number="5" >}}

Shortcode:

{{ range first (.Get "number") .Site.Taxonomy.tags[ .Get "tag" ] }} <= Is there any way?
  {{ .Render "teaser" }}
{{ end }}
index .Site.Taxonomy.tags (.Get "tag")