correctly detect a numeric color argument

I tried the 256 colors example in the README and got an error:

    alex$ ./colout.py '/var/[a-z]+' 135 </etc/passwd
    Traceback (most recent call last):
      File "./colout.py", line 92, in <module>
        lexers.append(lexer[1][0])
    IndexError: tuple index out of range

It seems some of the lexers are just empty tuples. I added a try/catch block around that line.