Restore selection after giving focus to prevent `clongeRange` errors

Fixes #106
Correct grammar
Improve example
Give focus to editor after applying selection instead of clicking beforehand

This fixes the tests in Firefox, and is probably the right thing to do regardless.
Create new range when restoring selection with `selection.selectMarkers`

This is needed because the editor might not have focus when restoring a selection, e.g. in the tests.
Fix failing Firefox test by using `scribe.setContent` instead of `scribe.insertHTML`
Tidy up
Re-use `selection` object
Remove redundant if condition
Tidy up