Add descriptor_value_by_name

We’ve added a feature to the global filters to look up a descriptor value by name. Here’s an example of how you would use it:

{% assign language = variant | descriptor_value_by_name: "Language" %}
{% if language == "English" %}
  Language: <img src="uk-flag.jpg" alt="English" />
{% else %}
  LOL I DUNNO?
{% endif %}

Note that it returns a falsy value if it isn't set OR if the variant doesn't have that descriptor at all.