Consider all overlapping exclusion rules, not only the first one

If the site (or another extension) introduces several keyboard shortcuts that the user wants to enable on different pages, it is natural to create a set of exclusions for different pages on that site.

For example:
```
*://site.com/* b
*://site.com/*.html r
```

It seems counter-intuitive to stop after the first match. This way on `*.html` pages `r` is not excluded at all. (Or `b` if the order is switched.)

This PR changes this behavior to looking through all exclusion patterns.