Consider more sophisticated "break opportunities"

Right now we consider line breaks possible where there is white space. A few details to consider are (1) handling of a full set of non-breaking spaces and (2) broader language support in languages where whitespace might be less common between words.

For future reference: Unicode defines line break opportunities in [this tech report](http://www.unicode.org/reports/tr14/). However, note we don't need to accommodate every possible line break so this is likely overkill for many languages. See also
https://github.com/devongovett/linebreak
https://github.com/niklasvh/css-line-break