Add a common configuration section.

Some new 2.2.1 settings were incorrectly added to the indexer section.
Setting these keys requires a new common configuration section. The
following keys were moved to this new section:

lemmatizer_base on_json_attr_error json_autoconv_numbers
json_autoconv_keynames rlp_root rlp_environment rlp_max_batch_size
rlp_max_batch_docs

See http://sphinxsearch.com/docs/manual-2.2.1.html#confgroup-common for
more information.