Add basic CMakeFile.txt for easy compilation
Add basic CMakeFile.txt for easy compilation

If someone just wants to build the executable, it is great to have a
simple procedure such as running `cmake . && make`

Additionally, you can include this into your own CMake-based project by
using `add_subdirectory(/path/to/cpp-markdown)` and then
`target_link_libraries(your_target PUBLIC cppmarkdown)`, which will use
the CMake settings (including CXXFLAGS etc.) from the parent project and
auto-configure include and link paths.

sevenjay/cpp-markdown,strivingboy/cpp-markdown