doc:add grn_plugin_proc_get_var and grn_plugin_proc_get_var_by_offset

TODO:

Translate it in doc/locale/ja/LC_MESSAGES/reference.po
Translate it in doc/locale/en/LC_MESSAGES/reference.po
doc:add grn_plugin_expr_var_init and grn_plugin_command_create

TODO:

Translate it in doc/locale/ja/LC_MESSAGES/reference.po
Translate it in doc/locale/en/LC_MESSAGES/reference.po
Merge pull request #178 from naoa/doc-plugin-api-reference

Add plugin API reference

Patch by Naoya Murakami. Thanks!!!