Add proxy groups config field and impl server selecter

[Hanaasagi]

1. Add groups field in config

groups:
  - name: "JP"
    type: load_balance
    proxies:
      - "Tokyo Sakura IPLC 01"
  - name: "HK"
    type: select
    proxies:
      - "HK vultr IPLC 01"

2. impl a server selecter with hash ring.