* Replace `structopt` with `clap`
* Change `port` options from `String` to `u16`
* Shorten clap options config
`name` and `long` use the filed name by default.
mini-redis uses the CLI derive pattern. Clap does not yet have a release
supporting this pattern. Using structopt allows mini-redis to avoid git
dependencies.
Continuation of #11. Refines the client structure and implements GET.
`clap` is decoupled from the lib code. This is done to avoid any CLI
parsing concerns to leak into the lib. The main motivation for this is
to allow the reader to focus on Tokio concerns and not CLI parsing
concerns.
