Easily configurable question prompt for Readline()

Sorry about opening bunch of issues, just trying to gather my thoughts around what a Inquirer.js clone in Go would look like.

Currently, to read input from the user without initializing a shell session, the code looks like this:

```go
sh := ishell.New()
sh.SetPrompt("First name :")
firstName := sh.Readline()
sh.SetPrompt("Last name :")
lastName := sh.Readline()
```

A more preferable way to work with the library would be like this:

```go
...
firstName := sh.Readline("First name")
lastName := sh.Readline("Last name")
```