Print doesn't display text if string doesn't end with '\n'

`fmt.Printf` doesn't work when there is no new-line character at the end of the string.

```go
// This works fine
shell.AddCmd(&ishell.Cmd{
Name: "hello",
Func: func(c *ishell.Context) {
fmt.Printf("Hello\n")
},
})

// This does not
shell.AddCmd(&ishell.Cmd{
Name: "hello",
Func: func(c *ishell.Context) {
fmt.Printf("Hello")
},
})
```