add defns of xid-start and xid-continue to parser-tools

r? @elibarzilay

This pull request adds xid-start and xid-continue lex-abbrev to parser tools. I'm submitting this as a pull request rather than as a simple commit because it adds a large-ish (800K) text file from the Unicode consortium from which these defns are extracted.  I figured the automatic push e-mail would be large enough to get people's attention, so I might as well get approval beforehand. 

Comments?