Read configs from a file

[sunglim]

Motivation:

The configurations are hard-coded. It makes us very difficult to update when the application is running.

The configs are hardcorded as below.

    go order.StrategryBuyEveryDayIfBelowAverage("12:00", []order.StrategryBuyEveryDayIfBelowOrder{
        {
            Code:     krxcode.Code농심홀딩스,
            Quantity: 1,
        },
...

Ideally, the configuration files should be passed as arguments. Also, the configurations should be reloaded if changed.

Technical decisions:

• Receive file name as argument - if file name is provided, other configurations passed as arguments are ignored

• Use https://github.com/spf13/viper which is widely used package for configuration

• Use YAML format because it's easy to read, comments are allowed

• [x] Read config file - for StrategryBuyEveryDayIfBelowAverage strategy

• [x] Reload app when config is updated

• [x] Read config file - for StrategryBuyEveryDayIfLowerThan strategy