Add workaround for dot-separated commands of Redis modules

[burre]

Hi there!

I ran into a problem when I tried to work with the RedisJSON module, because its commands consist of a prefix and the command itself, separated by a dot. Having looked at other similar modules, I found that they also use a similar approach.

But the resty/redis client implementation, based on the __index "magic" method, cannot handle such commands. Calling the method (function) through [] also did not bring results. So I did a little trick.

I propose to use the "underscore" (_) instead of the "dot" (.) when calling the method, and swap them back in the client code. For example:

local client = redis:new()

local ok, err = client:connect(host, port)
-- error checking goes here

client:json_set("key", "root", '{"key" : "value"}')

Perhaps this is too simple solution, but it works for me and, I hope, for someone else.

[spacewander]

Maybe you could introduce the namespace concept? Like this:

redis.register_module_prefix('json')
local client = redis:new()

local ok, err = client:connect(host, port)
-- error checking goes here

--  works like `redis.set`, but use a separate `redis.json` table and a `json.` prefix.
client.json.set("key", "root", '{"key" : "value"}')

[burre]

@spacewander Thanks a lot. It is a more complicated solution but more elegant as well.
I'ii try implement it.

[zhuizhuhaomeng]

#211
please use this feature in this PR.
@burre