Fix off-by-1 error in getblockcount
Add a `getblock` RPC
Support user configuration file

The file ~/.config/wizards-wallet/wizards-wallet.conf should look
like

````
[Bitcoin]
peer_port = 8333
peer_addr = "localhost"
rpc_server_port = 8001
rpc_server_addr = "localhost"
````

We should support multiple sections like this with [Bitcoin] replaced
with other network names (BitcoinTestnet is the only other one that
is supported for now), but we are held up on this until we sort out
https://github.com/alexcrichton/toml-rs/issues/9.