Code section comments with # get their own table of contents entry in the readme

For example:

````markdown
# Stuff you need to do

```bash
# Run this command if you want, but it's a good idea.
echo "hello"
```
````
`oclif-dev readme` will generate a TOC that contains this:
```markdown
* [Stuff you need to do](#stuff-you-need-to-do)
* [Run this command if you want, but it's a good idea](#run-this-command-if-you-want-but-its-a-good-idea)
```