Auto-generate sitemap

[audreyfeldroy]

Upon site generation, a sitemap should be auto-generated.

The sitemap should follow best practices:

• https://support.google.com/webmasters/answer/183668?hl=en
• http://www.sitemaps.org/protocol.html

If any manually created sitemap(s) exist in assets/, a message should be printed saying "Sitemaps already exist at and will not be auto-generated."