WIP: force blank line before code blocks

I want to move away from consuming any number of blank lines before the
code block to forcing a single blank line or the start of the file.

i.e. code blocks have to be like

```code
x = 1
```

blah blah blah.

and not like
```code
x = 1
```
blah blah blah.

question: do we force a blank line after the code block? I think so,
because this is what everyone does anyway and is what pandoc requires.

blank lines in excess of 1 will be considered as part of the markdown
cell.

This requires changes to the code block regex and maybe to
`MarkdownReader.parse_blocks`.