plugin:add initial setting API for command plugin

I added initial setting plugin API to make it easy to create a command plugin. 

refs [[groonga-dev,02344]](http://sourceforge.jp/projects/groonga/lists/archive/dev/2014-May/002344.html)

If this is merge, I will document it later.

Should I be modified to use this API in the following builtin plugins?

[groonga/plugins/ruby/eval.c](https://github.com/groonga/groonga/blob/master/plugins/ruby/eval.c)
[groonga/plugins/ruby/load.c](https://github.com/groonga/groonga/blob/master/plugins/ruby/load.c)
[groonga/plugins/ruby/ruby_plugin.h](https://github.com/groonga/groonga/blob/master/plugins/ruby/ruby_plugin.h)
[groonga/plugins/suggest/suggest.c](https://github.com/groonga/groonga/blob/master/plugins/suggest/suggest.c)
[groonga/plugins/table/table.c](https://github.com/groonga/groonga/blob/master/plugins/table/table.c)