Integrate a tool for catching missing includes

Let's integrate a tool such as:

* https://include-what-you-use.org
* http://clang.llvm.org/extra/include-fixer.html
* https://github.com/google/styleguide/tree/gh-pages/cpplint

to catch missing includes that might not appear until Travis tries to build your code for a different OS, see https://travis-ci.org/mapbox/node-cpp-skel/builds/394326432 for example and notice how this builds for macOS but not Linux (this was due to missing `#include `).

Referencing @springmeyer's comment and tool suggestions here: https://github.com/mapbox/node-cpp-skel/pull/129#issuecomment-398533472

Compare include-what-you-use, include-fixer, and cpplint, then inspect the Travis build referenced in the issue to see where a check could run. Done means the selected tool is integrated and catches missing C++ includes such as across the relevant builds.

Written by the indexing model from the issue text.