Add prompt option for IEx

closes #1946
I am thinking about the API.
What do you think about it? I would be grateful for any feedback. :)
(Maybe I should post to the mailling list?)

## API

The prompt options has two types.

* `:default` - used when `Node.alive?` returns false
* `:alive`    - used when `Node.alive?` returns true

The part of the listed in the following of the prompt string is replaced.

* `%counter` - the index of the history
* `%prefix`    - a prefix given by `IEx.Server`, for now "iex", "rem" or "pry"
* `%node`     - the name of the local node

### Examples

* `prompt: [default: "%prefix(%counter)>", alive: "%prefix(%node)%counter>"]`

this is same with the current prompt

```
iex(1)> :a
:a
iex(2)> :b
:b

iex(name@yuki_ito)1> :a
:a
iex(name@yuki_ito)2> :b
:b
```

* `prompt: [default: "iex>", arive: "iex>"]`

the prompt is always "iex>"

```
iex> :a
:a
iex> :b
:b

iex> :a
:a
iex> :b
:b
```