Order include directives alphabetically.

[syml]

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.

[dbarnett]

Having two partial formatters for C++ might mean people would want #44 so you could run this include sorting and clang-format on the same source code.

[fowles]

Discussion with clang-format owners indicate they may be willing to implement this in clang format.

[dbarnett]

Looks like clang has a sortIncludes option now. Does it sort includes how you want it automatically now, or would we need to explicitly configure something to enable it?