Wrap text outside of <p> in <p> when hitting enter when useLineBreaks==false

Suppose for some reason the body ends up like this (and it has for me):

```html
<p>Hey</p>
Just testing|
```

Currently, when hitting return with the caret after `Just testing`, we end up this this:

```html
<p>Hey</p>
Just testing<br>
|
```

Or worse (in Chrome), this:

```html
<p>Hey</p>
Just testing
<div>|</div>
```

With this fix, when `useLineBreaks == false`, the result will be a clean:

```html
<p>Hey</p>
<p>Just testing</p>
<p>|<br></p>
```
(The `<br>` is just there to give the `<p>` some height, exactly as Wysihtml5 currently behaves when hitting enter from within a `<p>`.)

It also works great with Shift-Enter, in which case the result would be:

```html
<p>Hey</p>
<p>Just testing<br>
|</p>
```