Add wallet commands

[NicolasMahe]

This PR add the commands related to wallet

➜ ./dev-cli wallet

Manage wallets

Usage:
  mesg-core wallet [command]

Available Commands:
  create      Create a new account
  delete      Delete an account
  export      export an account
  import      import an account
  list        list accounts

Flags:
  -h, --help   help for wallet

Global Flags:
      --no-color     disable colorized output
      --no-spinner   disable spinners

Use "mesg-core wallet [command] --help" for more information about a command.

[antho1404]

I did some feedback in #809 (and some refactoring) but nothing blocking and everything works great

[NicolasMahe]

@antho1404 Your modification has been merged and I added the pretty print for the export command