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.
sample config
rearrange lines
added comments
test merging plugin keys
Merge pull request #24 from mackerelio/config-include

config: support `include` key