Add support for list default value

[mwsteb]

First of all, thanks for a great gem!

Here's my attempt at implementing #66.

I wasn't sure how strict to be about default value type. For now, I opted for giving the user scissors and hoping they won't run with them.

Hope you don't mind I butted in like that, @tleish! Not sure if you started to work on this or not.

closes: #66

[tleish]

@mwsteb - I don't mind at all. While my example in #66 shows kredis_list, I want this with other (if not all) Kredis types. I might suggest adding more default types to this PR, or removing closes: #66 from the PR comment so additional PRs can be added with default types.

[mwsteb]

Makes sense. Since it's not a lot of code and it's all related, I can do it all in this PR.

I'm not sure about some types:

• Proxy - unclear what this type is exactly and how it's used and what a typical default value would be
• Cycle - setting the default value can be achieved by manipulating the order of the list - eg. Kredis.cycle "mycycle", values: %i[ three one two ]. Is adding an additional default option worth it?
• Slot(s) - seems a bit contrived to be using default value for this type, but maybe I just lack imagination :)

[tleish]

Proxy is a base class, so I don't think you need to worry about it. I assume it's safe to skip defaults for the other types as well for now.

[dhh]

Superseded by #89