Issue-105 Support time.Duration in viper.Unmarshal

[yauhen-l]

Just add DecodeHook to DecoderConfig.
Allows to unmarshal into struct with time.Duration fields:

package main

import (
    "bytes"
    "fmt"
    "github.com/spf13/viper"
    "time"
)

type Config struct {
    Timeout time.Duration
    String  string
}

func main() {
    var cfg Config

    viper.SetConfigType("yaml")

    var yamlExample = []byte(`
Timeout: 200ms
String: it works!
`)

    viper.ReadConfig(bytes.NewBuffer(yamlExample))
    viper.Unmarshal(&cfg)

    // {Timeout:200ms String:it works!}
    fmt.Printf("%+v\n", cfg)
}

[CLAassistant]

All committers have signed the CLA.

[jyellick]

This changeset looks very much like the code used in UnmashalExact.

Fixing Issue #105 is something I'd certainly like to see done, but could this patch be refactored so that Unmarshal and UnmarshalExact are combined to use the same codepath? This patch, as is, I believe will still produce errors in UnmarshalExact.

[awfm9]

I will try to take a look at this PR soon. In the meantime, if the suggestion by @jyellick could be implemented, that would be great.

[awfm9]

@yauhen-l it seems straight forward enough to add the hook to the UnmarshalExact as well, since it already uses a custom configuration. Could you add that?

[yauhen-l]

@awishformore, I will take a look in the evening

[yauhen-l]

@awishformore, @jyellick please review