Before this change, ReviewBot would try to install CppLint whether or not the user required it. This change removes CppLint from the required dependencies and creates an optional target 'all' that installs CppLint. This target can be extended in the future to include other dependencies. 
This change also updates the CppLint installation documentation to tell the user that they will need to manually install CppLint to use it.

pip install -e ./bot does not install CppLint

pip install -e ./bot[all] installs CppLint