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.
Oh wait... We're [copying the headers on request instantiation](https://github.com/cwinters/vegeta/blob/targets-dsl/lib/targets.go#L35) anyways. So this should be fine.