Extend the text format to contain node tags

In order to allow htools to make use of node tags, add them to the
text format. This is done by adding a new column at the end of the
node lines. If this column is missing, the default value (which
is the empty list) is left unchanged, thus yielding the current
behavior.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>