filetype: add support for cmake

We've been using CMake a little more over here, and I noticed that `CMakeLists.txt`` was not getting the filetype set correctly.  After digging in, I see that our setting for `*.txt` files is taking precedence over the core runtime's setting (because our's appears first, I suppose).  So, this adds the autocommand before our `*.txt` entry to make sure `CMakeLists.txt` is highlight correctly.