add option for user to customize file path

I like the simplicity of this gulp plugin, but I cannot customize the revisioned file path, so I added an option for that purpose.

usage:
```
gulp.src(SRC)
    .pipe(rev({ format: '{name}.v{hash}.{ext}'}))
    .pipe(gulp.dest(DEST));
```

the default naming format is respected, I also added test case for this modification