Order include directives alphabetically.

The google C++ style guide recommends to sort include directives alphabetically. https://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Names_and_Order_of_Includes

It would be nice if codefmt's FormatCode could also do this.

Start at the FormatCode entry point named in the issue and trace how code formatting is applied to C++ source. The work is done when include directives are sorted alphabetically in the formatted output, while the existing formatting behavior remains intact.

索引モデルが issue の本文から書いたものです。