Addition of linesBounded function to Data.Conduit.Text module

Hi Michael,
This is an attempt to provide a function that adds a guard
on the line length (when you can't be sure that the lines
will be reasonably small.) The function throws an exception
whenever its about to yield a line greater that a length given
by a paramater. This prevents assembly of chunks into
huge lines which could cause servers to choke.

I don't know if everything is correct so you had better
double and triple check this before you include this
(if you do ultimately decide you want it of course).
Its really just your function with a few lines added.
 It was thrown together in haste
in the midst of doing a million other things. I'm not really
conversant in conduit but I can appreciate that its a work of
art and what I understand of it impresses me greatly.

   Thanks, Mike W.