improve gen dao to support removeFieldPrefix

[zhangyuyu]

Given config the removeFieldPrefix as below,
When run gf gen dao
Then you can have the fields without prefix in struct(do/entity).

gfcli:
  gen:
    dao:
      - link: "mysql:user:pass@tcp(x.xx.xx.xx:3306)/xx"
        removePrefix: "t_"
        removeFieldPrefix: "f_"
        jsonCase: "Snake"

[hailaz]

#3205