Truncate lines in parser errors.

The offending lines of code displayed in the error message are truncated to 80
characters. This should make reading the error messages less of a pain when
dealing with very long lines of HTML/XML.

lib/oga/xml/parser.y

@@ -204,7 +204,13 @@ end
           prefix = '   '
         end
 
-        code << "#{prefix}#{number}: #{line.strip}\n"
+        line = line.strip
+
+        if line.length > 80
+          line = line[0..79] + ' (more)'
+        end
+
+        code << "#{prefix}#{number}: #{line}\n"
       end
     end