v0.41.0

Most relevant commits:

• compiler/ast: document that URL node also represents an URL in Markdown
• compiler/ast: rename Extends.Context field to Format with Format type
• compiler/ast: remove 'URL.Context' field
• templates/builtin: panic parsing invalid regexp instead of fatal
• templates/builtin: add unmarshalJSON builtin function
• templates/builtin: prefix errors with builtin name
• templates/builtin: panic string instead of error in regexp builtin
• templates/builtin: add 'form' builtin variable