Add a splitter for splitting on runs of Unicode whitespace

- The splitter should discard leading & trailing whitespace
- Add another splitter for splitting on ASCII-only whitespace?
    - Name the ASCII one "`SpaceSplitter`" and the Unicode one "`UnicodeSpaceSplitter`"
- Add a `read_*()` function that splits on whitespace?
- Add an option for translating to a single space?
- Don't break on non-breaking spaces?
- Define "space" by `str.isspace()`?