config: support `include` key

mackerel-agent includes additional config files if mackerel-agent.conf is like below:

    apikey = ...
    include = "/etc/mackerel-agent/conf.d/*.conf"
    ...

Any existing scalar/array keys are overwritten by included config files. Non-conflicting  table keys (map keys) are merged. Otherwise, undefined.
[IMHO]
I think it is better to test merging plugin settings that have the same keys.