custom user flags

**objective**
the program can simulate =
```
human vs human
human vs engine
engine vs engine
```

nice to have custom user flags before run the program, e.g.
```
chs -w human -b human
chs -w human -b engine --engine_path_b /bin/fruit
chs -w engine --engine_path_w /bin/sjaakii -b human
chs -w engine --engine_path_w /bin/crafty -b engine --engine_path_b /bin/sjeng
```