Wordpress code blocks (fixes #1186)

This is a basic fix for https://github.com/getnikola/nikola/issues/1186.  If you guys have tests I haven't modified them yet.

All it does is replace

```
[code language="stuff"]
code here
[/code]
```

with 

\`\`\`stuff
code here
\`\`\`

(no idea how to render \`\`\` inside of a code block on GitHub) in the Wordpress importer.