v0.40.0

Most relevant commits:

• compiler: initialize the named function return parameters
• cmd/scriggo: add builtins 'splitAfter' and 'splitAfterN' to serve
• template/builtin: add builtins to operate on time values
• templates/builtin: add 'marshalJSON' and 'marshalJSONIndent' functions
• compiler/lexer: parse URLs in Markdown context
• compiler/parser: fix error returned if a rendered partial does not exist
• all: implement function finalizer