Added #run method which doesn't write the files to disk

[normanrz]

I wanted to be able to execute the pipeline without writing the files to disk. So that I can post-process the files in another in-memory build tool like gulp.

[leeola]

@normanrz If i may ask, what is your workflow for using run() with Gulp? Do you have any examples of this? I'd be quite curious.

[normanrz]

Sure. I use gulp for preparing all of my assets and metalsmith only for the blog posts and static html pages. So, I built a bridge (transform stream) that passes files from gulp into metalsmith and back to gulp: https://github.com/scalableminds/metal-common/blob/master/lib/gulp_bridge.coffee

Then I use it with something like

gulp.src("src/blog/**/*")
  .pipe(gulpBridge((metal) ->
    metal
      .use(markdown())
      .use(templates("handlebars"))
  ))
  .pipe(gulp.dest("dist"))