Adds taxonomies and collection assigned taxonomies to sitemap

[mikemartin]

Hey @robdekort I had a need for this and it seemed like a relatively simple PR. Let me know your thoughts.

Sitemap global blueprint:

• Adds Taxonomies field to output taxonomy term urls
• Adds Collection Taxonomies field to output taxonomy term urls by collection

Sitemap.xml template

• Adds taxonomy tag to output taxonomy term urls by handle
• Adds taxonomy tag for each specified collection to output collection term urls

[robdekort]

Very nice. I like it! Thank you Mike.