Add name/names requirement for async commons chunk

[andrewdushane]

To use the async option with the CommonsChunkPlugin in the current version of webpack (3.0.0, going back to 2.5.0), a name or names option which matches entry name/s is also required. This PR changes the code example for async commons chunk to show that.

[andrewdushane]

Travis is failing on lint:links and seems to be choking on a link to https://github.com/webpack-contrib/file-loader/edit/master/README.md
From the log:

Guessing --root from input files: file:///home/travis/build/webpack/webpack.js.org/build/
should respond with HTTP status 200
502 https://github.com/webpack-contrib/file-loader/edit/master/README.md at build/loaders/file-loader/index.html:19:59413 <a class="page-links__link" href="https://github.com/webpack-contrib/file-loader/edit/master/README.md">...</a>

[skipjack]

@andy-j-d I think what you saw before was just a temporary link failure (these happen fairly often) -- the other errors that cropped up after weren't related to this PR and were fixed in #1355.

[skipjack]

Thanks for your contribution, looks great! I'll just need to coordinate merging this, #1370, and #1428 as I think there's some overlap. Should be merged soon.