Allow passing directories as the config sources

We should allow passing the entire directories as the config sources. In
this case CouchDB will traverse the given directories and load every
*.ini file found.

E.g.

"... -couch_ini /etc/couchdb/default.d/ /etc/couchdb/local.d/ /path/to/extra/couchdb/default.ini /path/to/extra/couchdb/local.ini ..."

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Order matters since otherwise we'll write all the changes to the first config in chain and it is default.ini.