Rewrote Parser, using regular expressions instead of walking though

Rewrote Parser, using regular expressions instead of walking though
the string one line at a time.  The resulting code is (in my opinion,
anyway), much easier to read.  In the process, I found and fixed a
bug in the orginal parser's line numbering in error messages (it was
inconsistant between 0-based and 1-based).  Also, check for missing
blank lines after the prompt on all prompt lines, not just PS1 lines
(test added).