Check for undefined or null on contentDocument

Set default parameter on contentDocument
Merge pull request #37 from Rise-Vision/fix/fix-contentDocument-check

Check for undefined or null on contentDocument
Add function to add CSS rule into document

Enable adding multiple CSS rules at once

Switch to an array for addCSSRules parameter
Merge pull request #38 from Rise-Vision/chore/add-css-rule

Add function to add CSS rule into document