Maintain backward compatibility with existing target files

Maintain backward compatibility with standard target files by peeking ahead to the next value after reading a method + URL; if the next line starts with another HTTP method then short-circuit the header/body parsing and remember the peeked value for the next call to create a Target.

FWIW, I used `Peek()` as the metaphor for the lookahead Scanner because the Golang `text.Scanner` does it that way.