Explore automatically paging long output

[tleonhardt]

It would be nice if long output which is bigger than can fit within the height of the terminal is automatically paged or at least if the option for doing so in an opt-in fashion existed within cmd2.

The size of the terminal can be retrieved on all platforms as follows:

import os
height, width = os.get_terminal_size()

It probably wouldn't make sense to add a dependency on Click just for this, but Click's echo_via_pager() function provides nice pager support. Perhaps we could use the underlying implementation for Click's pager support as an inspiration.

We would need to make sure we deal appropriately with cases where stdout is redirected/piped so that we never attempt to use a pager in those cases.

[tleonhardt]

PR #297 adds a new cmd2.Cmd.ppaged() method for displaying long output using a pager.

On macOS and Linux it uses less -SRXF and on Windows it uses more.

If we actually wanted to in some cases redirect ALL long output through a pager, we could add a new runtime-settable attribute to turn this capability on/off.