Improvements to Split block for whitespace and line options

* Split by whitespace now uses the built-in definition of whitespace \s
  This catches all characters defined as whitespace, see below:
  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp
* Split a line by all unicode compliant line breaks. The biggest impact here is
  that OSX and Windows files will now split the same way.
The cr option is still around, but there's no longer a need for it, IMO.

@brianharvey this is a fix for the recent Piazza issue: https://piazza.com/class/h8yx482idav52y?cid=353

As an aside, I think the ordering of the split menu options should be reordered
to be alphabetical. I can do that, if that sounds good. 

Also (forgot to put it in the commit message, but) I fixed a couple typos in 
the error strings of the split block.