Update dependencies to latest versions.

change field size threshold to 1000

add caddyfile directive hash_dir

1. for post fields larger than 10000byte, store its md5 value in log
   file and its raw content in hash_dir.
2. if the value of directive hash_dir is set to `on`, md5 values stores
   in default dir `hashdata` otherwise this value mean the customized
   dir.

update README.md