Add support for slices when using environement variables

[TheoBrigitte]

Fixes: https://github.com/spf13/viper/issues/1732

Following up on @lwlee2608 great work from https://github.com/spf13/viper/pull/1733, here is a PR which adds support for using slices with environment variables and provide an additional fix to ensure we are not limited by the original config slice length.

Here is an example yaml config file :

clients:
  - name: foo
  - name: bar
  - name: baz

Values can now be overriden using

CLIENTS_0_NAME=one
CLIENTS_1_NAME=two
CLIENTS_2_NAME=three

Note that for this solution to work the config must be initialized as folow:

    var yamlConfig = []byte(`
clients:
  - name: foo
  - name: bar
  - name: baz
`)

    viper.SetConfigType("yaml")
    r := strings.NewReader(string(yamlConfig))
    viper.ReadConfig(r)
    SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
    AutomaticEnv()
    // Configuration must have the same structure as yamlConfig
    var config Configuration
    v.Unmarshal(&config)

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[sagikazarmark]

Thanks @TheoBrigitte ! I'm gonna need some time to review this, but it looks good.