Support custom name matchers

[abemedia]

Fixes #89 and #208 in a more dynamic way than #81, #176 or #207. Rather than just supporting a single use-case this also allows any other form of custom name matcher e.g. using https://github.com/iancoleman/strcase to automatically match to snake_case or kebab-case.

For example:

type Target struct {
  FooBar string
}

var out Target

config := &DecoderConfig{
  Result: &out,
  MatchName: func(mapKey, fieldName string) bool {
    return strcase.ToCamel(mapKey) == fieldName
  },
}

decoder, _ := NewDecoder(config)

decoder.Decode(map[string]interface{}{
  "foo_bar": "baz",
})

Would result in out being:

Target{
  FooBar: "baz",
}

[mitchellh]

I love this. Thank you.

[abemedia]

You're very welcome. And thank you for creating such a great library!