Implement scripting API

[mijicd]

Extracted from this comment authored by @regis-leray.

Commands

• [x] EVAL script numkeys key [key ...] arg [arg ...] Execute a Lua script server side
• [x] EVALSHA sha1 numkeys key [key ...] arg [arg ...] Execute a Lua script server side
• [ ] SCRIPT DEBUG YES|SYNC|NO Set the debug mode for executed scripts.
• [x] SCRIPT EXISTS sha1 [sha1 ...] Check existence of scripts in the script cache.
• [ ] SCRIPT FLUSH Remove all the scripts from the script cache.
• [ ] SCRIPT KILL Kill the script currently in execution.
• [x] SCRIPT LOAD script Load the specified Lua script into the script cache.

Tips

• Define the API in zio.redis.api.
• Command-specific options should be defined in zio.redis.options.
• Additional inputs and outputs can be defined in zio.redis.Input and zio.redis.Output, respectively.
• Redis API evolves, therefore the commands listed above might be slightly changed. Double check the official docs before starting with implementation.
• Use telnet to understand command protocol details.
• Write inputs and outputs tests in zio.redis.InputSpec and zio.redis.OutputSpec, respectively.
• Write integration tests by expanding the zio.redis.ApiSpec.
• If you feel that the pull request size is growing out of control, feel free to split it but make sure to link this issue in each of the related PRs.

[mijicd]

(Partially) implemented by @anatolysergeev in #269

[drmarjanovic]

@mijicd, can I work on this one?

[mijicd]

All yours!