v0.3.7

• routes now accepts Url instead of &Url (Breaking)
• Improvements to fetch API
• Added raw!, md!, and plain! macros that alias El::from_html, El::from_markdown,
  and El::new_text respectively
• Attrs! and Style! macros can now use commas and whitespace as separators,
  in addition to semicolons
• Fixed typos in a few attributes (Breaking)
• Fixed a bug where an HTML namespace was applied to raw html/markdown elements
• New conditional syntax added in class! macro, similar to Elm's classList
• Listener now implements MessageMapper
• El methods add_child, add_style, add_attr, and set_text now return the elements,
  allowing chaining
• Fixed a bug with set_text. Renamed to replace_text. Added add_text, which adds
  a text node, but doesn't remove existing ones. Added add_class. (Breaking)