efficiency

```
cleaner <- xml2::read_xml(system.file("xslt/justthetext.xslt", package="hgr"))
```

needs to be moved to `.onLoad`/`.onAttach` and put into the pkg env so it's not read every time.

the object needs a validity check before use in the event stuff got gnarly on the user-side and we're left with a dangling `XPtr` (thx, `xml2`).
