add ignoreCollapse options

[jamuhl]

Based on the specification this module collapses nodes only having whitespace. While this is correct there are cases you want to persist those. We built a simple syntax highlighting for i18n formats based on this module where we overlay a div over a text input field - but having this collapsing the content gets out of sync -> therefore, this option would help:

// content
<p>
Some
</p>
<p>
Strange Value2
</p>

// ends in (without this option)
<p>
Some
</p> <p>
Strange Value2
</p>