Always flatten callback arguments.

Closes #133.

File                             Raw         Min         Zip     Min/Zip
index.js                 16053/15.68  5270/05.15  4335/04.23  1891/01.85
Change brace style.
File                             Raw         Min         Zip     Min/Zip
index.js                 14856/14.51  5240/05.12  3220/03.14  1875/01.83