Support nested options

[sttk]

This function will be used in vinyl-fs to solve the issue gulp#2288.

[phated]

Unfortunately this won't solve the sourcemap issue. I will try to follow up on that when I have some free time.

[sttk]

@phated This pr only enables to specify advanced sourcemap options.
Moreover, we need to modify vinyl-fs and vinyl-sourcemap to write outputs by these options in source map files.

gulp.src('src/**/*.js', {sourcemaps: true})
  .pipe(gulp.dest('dist', {
    sourcemaps: { destPath: '.',  sourceRoot: '../src' }  // Enable to specify nested options like this.
  })); 

At https://github.com/gulpjs/vinyl-fs/blob/master/lib/dest/sourcemap.js#L11, these nested options can be gotten like following.

var srcMap = optResolver.resolve('sourcemaps', file);  // => { destPath: '.',  sourceRoot: '../src'  }

[3cp]

Just a thought, no matter how is it fixed underneath, in this surface API:

gulp.dest('dist', {
    sourcemaps: { destPath: '.',  sourceRoot: '../src' }
  });

The sourceRoot is unnecessary, because you can automatically figure out sourceRoot based on the first arg to gulp.dest which is 'dist', plus the original file.base in vinyl-fs file itself.

I would suggest gulp to get the sourceRoot right by default for both gulp.dest('dist', {sourcemaps: true}) and gulp.dest('dist', {sourcemaps: '.'}).

[phated]

I think if we are supporting nested options, we are complicating our options too much. Gulp was never intended to have the complete set of gulp-sourcemaps; it is only the initialization. End-users are expected to combine other plugins for advanced features.

[phated]

@sttk I created a new tool for updating the repository scaffold. I'll need to show you that some time.