[error] Support fmt.Formatter interface

[at15]

I am used to log.Warnf("err %v", err), but since we didn't implement formatter interface, the result is just struct ... it's quite a pain to call err.Error everytime

• https://golang.org/pkg/fmt/#Formatter

type Formatter interface {
        Format(f State, c rune)
}

• [ ] does %w need some extra handling?
• [x] print string #63
• [ ] print stack
• [ ] pass format to inner error and multi error
• [ ] it seems non standard formatter is also possible?

[gaocegege]

强无敌

[at15]

the print interface seems also changed with new x/errors #109

[at15]

Although it is important ... won't hurt without its full implementation ... so moved it to 0.0.14