Paginate CLI prefix listing output

Change the way the CLI works so that we fetch small chunks of the result
and output iteratively instead of fetching everyting in a big chunk.

Gives more instant feedback to the user that the result is being
fetched.

In addition, this fixes a bug where result sets over 1200 prefixes would
not be displayed (max_result was set to 1200).

Part of #278
CLI pagination for VRF list

Also fix bug in nipapd where it didn't use the 'offset' option at all
for listing VRFs

Part of #278
Update README.rst
CLI pagination for pools

Last and least, add pagination for listing pools. This also fixes
listing more than 50 pools which previously was not possible due to the
limit being set to low.

Fixes #278
Merge branch 'master' of github.com:SpriteLink/NIPAP
Merge branch 'cli-paginate'