add option to display less on alternate screen

[thomaswuensche]

Currently, Pry passes the -X option to less in the class SystemPager, which 'Disables sending the termcap initialization and deinitialization strings to the terminal'. This means that paging happens on the main screen and its contents will stay there after quitting less.

For me, having less displayed on the alternate screen of the terminal is very useful, because it de-clutters my main screen. For example, if I look at an API response in a Pry session, I often page through a lot of data and then return to the REPL, but whatever I was looking at before is now overwritten by the contents of the API response. By displaying less on the alternate screen, I can quickly return to the previous context.

However, I don't want to change any existing behaviour or already established workflows, so this PR adds a config option.
This way, one can just set Pry.config.less_alt_screen = true in .pryrc and less will be displayed on the alternate screen.