Default fenced code block should assume plain text

[eed3si9n]

This is an offshoot of #64 and lightbend/ssl-config#51.

steps

Use Generic theme and put the following in the body:

```
curl https://financialcryptography.com # uses cacert.org as a CA
```

problem

The syntax highlighting here is too aggressive, and doesn't really make sense to me.

This also manifests itself when there are single quotes used in the middle of the text as apostrophe.

expectation

Kramdown / Redcarpet or whatever that implements GFM uses ``` to indicate plain text without any special postfix. We should do the same.

https://help.github.com/articles/creating-and-highlighting-code-blocks/

/cc @dwijnand, @jonas