Fixed line breaks in rendered markdown output

[tobias-kuendig]

This PR fixes line breaks in the output of the markdown renderer.

Currently, if a markdown document is parsed and rendered using this markdown library, some relevant line breaks are lost:

• double line breaks after paragraphs

There are other (cosmetic) line breaks that are lost, but don't affect the validity of the markdown output:

• line breaks after headings
• line breaks after lists

There are some line-breaks that are added that should not be added:

• A new line at the end of code blocks is added.

Example

Input:

# heading

Paragraph 1

Paragraph 2

- List item 1
- List item 2

Paragraph 3

\```
some code
\```

Is currently rendered as:

# heading
Paragraph 1
Paragraph 2
- List item 1
- List item 2
Paragraph 3
\```
some code

\```

Details

func TestPR(t *testing.T) {
	input := markdown.Parse([]byte(`
# heading

Paragraph 1

Paragraph 2

- List item

Paragraph 3
`), nil)

	fmt.Println(string(markdown.Render(input, NewRenderer())))
}

Note that paragraphs 1 and 2 are now no longer two separate paragraphs. It is also common to add double line breaks after a list and a heading for better readability.

This PR ensures, that there are always two line breaks after a heading, a paragraph or a list.

[kjk]

thanks