Add NewStderrWriter

[c-bata]

Add NewStderrWriter to support stderr output. To close #101

Usage is like this:

package main

import (
	"fmt"

	"github.com/c-bata/go-prompt"
)

func completer(in prompt.Document) []prompt.Suggest {
	s := []prompt.Suggest{
		{Text: "users", Description: "Store the username and age"},
		{Text: "articles", Description: "Store the article text posted by user"},
		{Text: "comments", Description: "Store the text commented to articles"},
		{Text: "groups", Description: "Combine users with specific rules"},
	}
	return prompt.FilterHasPrefix(s, in.GetWordBeforeCursor(), true)
}

func main() {
	in := prompt.Input(">>> ", completer,
		prompt.OptionWriter(prompt.NewStderrWriter()))  // <- set stderr writer
	fmt.Println("Your input: " + in)
}

[c-bata]

Deprecate NewStandardOutputWriter because it's a little bit long :(

[rhysd]

Looks good for me. Please add 'Closes #101' to description of this PR or commit message since this PR resolves the same problem as what #101 wants to resolve.

[c-bata]

Thanks. I changed a description of this PR. I'll merge this.