Add a splitter for splitting on runs of Unicode whitespace

[jwodder]

• 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()?