add redis_stat.py

support keyspace anslysis

    $ time ./redis-rdb-tools/rdbtools/cli/redis_stat.py -k 'user-' -k 'file-' dump.rdb.part
             -         cnt  cnt_hash        cnt_string           mem        mem_hash        mem_string      z_cnt_no_expire       z_sum_expire
          user      220178    220178                 0      34886519        34886519                 0                    0     -81039445726.0
          file       71356         0             71356      15250363               0          15250363                    0     -26377119006.0
     z_unknown        2314         0              2314        252226               0            252226                 2314                  0
        zz_all      302813    220178             82635      52746444        34886519          17859925                 2314 -1.10728956618e+11