Add .bloom-text-for-css to enable text-sensitive CSS rules.

CSS normally can't get at the text in order to, for example, show something different if it is empty.
With this, you can add .bloom-needs-data-text to a bloom-translationGroup in order to get
its child bloom-editable's to have data-texts attributes and then write css rules like this one, which shows a word-breaking symbol only if the prefix isn't empty:
    .prefix .bloom-editable:not([data-text=""]):after{ content:'-';  }
Merge pull request #210 from BloomBooks/bloom-text-for-css

Add .bloom-text-for-css to enable text-sensitive CSS rules.