Add basic main CLI

I'm a visual type of person, I like to watch things evolve on the
command line. So I've added a basic main.rs to the project to watch
things on the command line as they evolve.

src/lib.rs

@@ -1,2 +1,6 @@
-pub mod cell;
-pub mod board;
+mod cell;
+mod board;
+
+pub use board::Board;
+pub use board::Point;
+pub use cell::CellValue;