Make middlewares optional

[matheusd]

This makes the middlewares offered by the server package optional. This
allows full customization by implementations.

Fixes #59

[patrick-ogrady]

@matheusd Just an FYI that it won't let me review this until you sign your commit.

[patrick-ogrady]

Example logs look great:

2020/07/24 13:58:36 Listening on port 8080
2020/07/24 13:58:51 POST /network/list 189.853µs
2020/07/24 13:58:51 POST /network/status 250.172µs
2020/07/24 13:58:51 POST /network/options 110.503µs
2020/07/24 13:58:51 POST /block 294.214µs
2020/07/24 13:58:51 POST /block/transaction 84.845µs
2020/07/24 13:58:51 POST /block 100.8µs
2020/07/24 13:58:51 POST /block 97.317µs
2020/07/24 13:58:51 POST /block 133.419µs
2020/07/24 13:58:51 POST /block 251.101µs
2020/07/24 13:58:51 POST /block 149.209µs
2020/07/24 13:58:51 POST /block 129.884µs
2020/07/24 13:58:51 POST /block 446.035µs
2020/07/24 13:58:51 POST /block 451.176µs
2020/07/24 13:58:51 POST /block 314.461µs
2020/07/24 13:58:51 POST /block 222.318µs
2020/07/24 13:58:51 POST /block 220.745µs

[patrick-ogrady]

Awesome work @matheusd 🎉

[matheusd]

Should've mentioned in the original msg, but here's something that can produce customizable logging with slightly more info (status code, total bytes sent): https://github.com/matheusd/middlelogger/