Add new `find-text` command

It'll emulate the search functionality (ctrl+f) on web browser.

The call is as follows:

```js
window.find("installation", false, true, true, false, true, false)
```

Arguments are:

* string to find
* aCaseSensitive
* aBackwards
* aWrapAround
* aWholeWord
* aSearchInFrames
* aShowDialog
