Add automated cpplint testing of pull requests

Like TheAlgorithms/Python#218 but for C++.  Enables Travis CI to run https://github.com/cpplint/cpplint on every pull request.

The owner of the this repo would need to go to https://github.com/TheAlgorithms/C-Plus-Plus and kick off the first build to enable free automated flake8 testing of each pull request.  Flake8 will find bugs and code smells.

https://docs.travis-ci.com/user/languages/cpp/

Author: cclauss

.travis.yml

@@ -0,0 +1,4 @@
+# https://docs.travis-ci.com/user/languages/cpp/
+language: cpp
+install: pip install cpplint
+script: cpplint .