Change single quoted strings that contain variables check to a warning.

This is actually not a MUST in a styleguide and there are legitimate
reasons where single quoted strings might contain the string '${'.
Anything to do with shell variables is a prime example:

```shell
'some command "${variable}"'
```
or for fancier things

```shell
'some command "${variable:"default"}"'
```

Still probably a good idea to warn people about potential issue, so warnings.