Added '--char-offsets' flag for token text in ConLL format

The concrete_dump.py script normally uses the token text stored in the
Concrete Tokenization objects.  The command line flag '--char-offsets'
adds a column to the 'ConLL-like' output that prints the token text
specified using the (optional) Concrete TextSpan for each token.
These TextSpans use character offsets for the Communication's 'text'
field.