Have 'getblock' work with 8 or more characters of a block hash prefix.

Previously it required the entire block hash.

We could always do this:

    $ cc getblock ec64deeb7f1295216f20ce5dbe68b0bd28189a5a644a111e722c05451d51e66c |
      grep -E '"(height|hash)"'
        "hash" : "ec64deeb7f1295216f20ce5dbe68b0bd28189a5a644a111e722c05451d51e66c",
        "height" : 175000,

Now we can do this too:

    $ cc getblock ec64deeb |
      grep -E '"(height|hash)"'
        "hash" : "ec64deeb7f1295216f20ce5dbe68b0bd28189a5a644a111e722c05451d51e66c",
        "height" : 175000,

I made it insist on at least 8 characters:

    $ cc getblock ec64dee |
      grep -E '"(height|hash)"'
    error: {"code":-1,"message":"Please provide at least the first 8 digits of the block hash"}

It does a linear search through all the blocks in the map, so it's a little slow - but if you don't know the full block hash it's better than nothing.