add run_demo.sh

fluid/machine_reading_comprehesion/DuReader/run_demo.sh

+python  run.py   \
+--trainset 'data/demo/trainset/search.train.json' \
+--devset 'data/demo/devset/search.dev.json' \
+--testset 'data/demo/testset/search.test.json' \
+--vocab_dir 'data/demo/vocab' \
+--use_gpu true \
+--save_dir ./models \
+--pass_num 10 \
+--learning_rate 0.001 \
+--batch_size 8 \
+--embed_size 300 \
+--hidden_size 150 \
+--max_p_num 5 \
+--max_p_len 500 \
+--max_q_len 60 \
+--max_a_len 200 \
+--drop_rate 0.2 \
+--log_interval 1 $@\