Support customising mapstructure.DecoderConfig for Unmarshal

[krak3n]

Hi

I recently ran into an issue when using viper.Unmarshal in-conjunction with Environment variables. I wished to pass a json object as a value to an Environment variable for user names and passwords that I didn't want to write to a config file, for example:

export MY_CONFIG_VAL={\"foo\":\"bar\",\"fizz\":\"buzz\"}

Whilst viper.GetStringMapString would work perfectly, Unmarshal would fail with:

* 'my_config_val' expected a map, got 'string'

I did some digging and saw this was failing in mapstructure. I saw it supported custom DecodeHook functions so I could write my own function to parse the value correctly, however viper does not support the ability to customise the mapstructure.DecoderConfig.

So this PR adds the following (it's worth noting this is 100% backwards compatible thanks to variadic arguments):

• Added a new DecoderConfigOption type allowing the user to write custom
  functions that can override the default mapstructure.DecoderConfig
  settings

• Added a new DecodeHook function which returns
  a DecoderConfigOption. This allows the user to easily set their own
  Decode hooks when Unmarshaling

• Updated Unmarshal, UnmarshalKey and defaultDecoderConfig to support variadic
  trailing DecoderConfigOption functions to allow for customisation of
  the default mapstructure.DecoderConfig

• Added a test case with example usage

I look forward to your review and feedback 😄

p.s and thank you for this wonderful library.

Edits: spelling

[CLAassistant]

All committers have signed the CLA.

[krak3n]

Also I believe this may also be a solution to #449 . Since the user could instead customise the default existing configuration instead, for example:

func ViperStructTag() viper.DecoderConfigOption {
	return func(c *mapstructure.DecoderConfig) {
		c.TagName = "viper"
	}
}

And then calling Unmarshal with the function above as an Option:

type config struct {
	Foo string `viper:"foo"`
}
var C config
viper.Unmarshal(&C, ViperStructTag())

Edit: Referenced wrong PR

[krak3n]

@spf13 @bep any feedback on this one?

[krak3n]

😭

[krak3n]

yey! thank you @bep