Add a `verbose` option

[paul0403]

It would be nice to have a verbose option to show all the history as we search through the tree:

>>> make_24.make_24(1, 2, 3, 4, verbose=True)
>>> 
Searching (1, 2, 3, 4)
  Consuming combination index (0,1)
    Visiting 1+2 -> 3
    Searching (3, 3, 4)...
       ...

    Visiting 2-1 -> 1
    Searching (1, 3, 4)...
       ...

    Visiting 1*2 -> 2
    Searching (2, 3, 4)...
       ...

    ...

  Consuming combination index (0,2)
     Visiting 1+3 -> 4
     Searching (4, 2, 4)...