Support CPM

It would be great if you project supported CPM: https://github.com/TheLartians/CPM.cmake

Then people who wanted to use it could conveniently refer to it in the CMakeLists.txt:
``` 
CPMAddPackage(
  NAME cpp-text-table
  GITHUB_REPOSITORY haarcuba/cpp-text-table
  GIT_TAG origin/master
)
``` 