Create a dedicated package for config management

[pedromotita]

Description

Instead of calling config from the CLI layer, create a dedicated package. Viper would be wrapped in this package, and would not appear anywhere in the code, all config management would be done by the config package.

It's API could look something like:

type PorkbunAPIConfig struct {
    Key       string `json:"key"`
    SecretKey string `json:"secretKey"`
    Domain    string `json:"domain"`
    Subdomain string `json:"subdomain"`
    Type      string `json:"type"`
}

type Config struct{}

func (c *Config) Get() (PorkbunAPIConfig, error)
func (c *Config) GetKey(key string) (PorkbunAPIConfig, error)
func (c *Config) Set(key, value string) error
func (c *Config) Delete(key) error

[paulopacitti]

I think it's a great refactor from what we have now, great suggestion!