improve init command to create v2/v1 yaml file

[dangquangdon]

sqlc init now would generate v2 format yaml file by default, user also can use --v1 to generate v1 format if needed.

Create an empty sqlc.yaml settings file

Usage:
  sqlc init [flags]

Flags:
  -h, --help   help for init
      --v1     generate v1 config yaml file
      --v2     generate v2 config yaml file (default true)

[kyleconroy]

Thanks! This has been on the todo list for a while.