Enhance foreach tag to support dynamic variables

[aerni]

This PR adds the option to pass a variable by parameter. This makes it possible use dynamic variables:

{{ foreach :array="fields[field].options" }}
    ...
{{ /foreach }}

[jasonvarga]

Made a little tweak here, but otherwise 👍 Thanks!

You introduced a small issue where if you did {{ foreach:unknown }}..{{ /foreach:unknown }} it'll actually try to use the string "unknown" for the loop.