Add rudimentary drop_tile and interactive mode

Adds basic drop_tile implementation. Though doesn't check for wins

Adds an interactive mode for testing drop_tile, which allows the player
to input which tile-space to drop on.

main.c

@@ -5,6 +5,12 @@
 int main(int argc, char *argv[])
 {
   struct Board *my_board = make_board(6, 7);
-  print_board(my_board);
+  while (1) {
+    print_board(my_board);
+    int drop_spot = 0;
+    printf("Where to drop? ");
+    scanf("%d",&drop_spot);
+    drop_tile(my_board, drop_spot-1);
+  }
   return EXIT_SUCCESS;
 }