Parse GV files rather than lines; allows multi-line GV assertions.

Changes of note:

  - GV assertions must now end with full stops.
  - GV files can contain line comments, beginning with --
  - GV assertions can span multiple lines.
  - Files can be specified on the gvi command line, like so:

      ./gvi file1 file2 ...

    To get a REPL, add "-i" or specify no files.