Helper for looping over sequences, particular in templates.
Often in a loop in a template it's handy to know what's next up,
previously up, if this is the first or last item in the sequence, etc.
These can be awkward to manage in a normal Python loop, but using the
looper you can get a better sense of the context. Use like::
>>> for loop, item in looper(['a', 'b', 'c']):
... print loop.number, item
... if not loop.last:(more...)