Add support for case insensitive wild card matching

```
a[href*="ha"] @href
```

will work and return the links that have 'ha' in the actual url but not 'Ha'. If I also wanted both / case insensitive matching, I'd need to do something like this:

```
a[href*="ha" i] @href
```

but this currently gives an error

```
XPath error : Invalid type
```

https://www.w3.org/TR/selectors-4/#attribute-case