cameo: fixed processing of omitted file names, documented file name usage

- README: document that file names can contain almost anything
- lang.l: reset the scanner state on \n, so that we stop looking for a
  file name if the file name has simply been omitted

cameo/README

@@ -48,6 +48,19 @@ File output:
 Writes all loaded paths in gnuplot format to the specified file. If the
 file name is omitted, "write" writes to standard output.
 
+File names can contain spaces and any printable characters, but no leading
+or trailing spaces. It is not possible to place a comment after a file
+name or at a place where a file name could be.
+
+E.g.,
+
+  write foo.gp   # wrong
+
+but
+
+  write foo.gp
+  # right
+
 
 Units: