add blank line before/after preformatted blocks

[chrispy-snps]

Fixes the preformatted block part of #176.

A blank line is inserted before and after preformatted blocks. Unit tests are updated.

For example, this:

import markdownify

md = markdownify.MarkdownConverter().convert

print(md(html="1<pre>2</pre>3<pre>4</pre>5"))

results in this:

1

```
2
```

3

```
4
```

5

[chrispy-snps]

@AlexVonB - I requested a review.

This change is subjective - most Markdown parsers will handle preformatted blocks with content lines directly adjacent to the fence lines. However, that style can be visually disconcerting for humans to read.

In my opinion, the improved readability is worth the extra two characters per preformatted element. (After all, isn't the purpose of Markdown to be a structural format that is also human-friendly?)

[AlexVonB]

Same as with #178, lgtm but we need to keep the general whitespace in mind. Thanks for tackling this! :)