Implement fmt.Stringer

[samb1729]

TomlTree currently has a ToString method but no String method which means it doesn't implement the standard interface for stringifying. I'm looking to be able to define an interface:

type Config interface {
    Get(key string) interface{}
    Set(key string, value interface{})
    fmt.Stringer
}

and use TomlTree as the implementation.

[pelletier]

Thank you for your contribution! PR merged.