handle overflow gracefully

When I edit a md text that is wider than the editor window, then this text is not shown/cut off in the preview.
Also when I have a md text that is very long than it takes up the whole page.

Current:

I propose to make the following changes to `martor.bootstrap.min.css`:

```
.main-martor {
margin: 1em 0;
position: relative;
overflow: hidden;
width: 100%
}

div.martor-preview {
padding: 1rem;
overflow: scroll;
background: #fdfdfd;
max-height: 50vh
}
```

This does the following:

- `max-height: 50vh` limit the preview height to 50vh (to avoid occupying the whole page - one can still use the full screen mode if needed
- `overflow: scroll` and `overflow: hidden` make the preview scrollable if it is too long
- `width: 100%`: let martor occupy the whole column instead of floating in the middle

Proposed solution: