UnifiedHighlighter Separator-based BreakIterator should work with Strings, not just a single character

For the `hl.bs.type` choice of SEPARATOR, it would be nice if we could support not just a single character, but a string.  In looking at the code, I see no reason Strings can't be supported other than a few signature changes on some constructors.

My use case: I have docs that I have section and page markers that make for conveniently-sized passages for highlighting, but there really isn't any clean way to mark those sections with a single character.  For instance, Tika will extract and mark pages with `<div class="page"><p/>....</div>`.  If I could pass in that `<div class="page">` tag as my separator, I could then just highlight within a page.