Update Prompt type form string to fmt.Stringer.

[luopengift]

On branch master Your branch is up to date with 'origin/master'.

Changes to be committed: modified: example/readline-demo/readline-demo.go modified: example/readline-im/readline-im.go modified: example/readline-multiline/readline-multiline.go modified: example/readline-pass-strength/readline-pass-strength.go modified: example/readline-remote/readline-remote-server/server.go modified: operation.go new file: prompt.go modified: readline.go modified: runebuf.go modified: std.go