Feature: navigate back and forth

[bAndie91]

this pr adds a 'View' menu to the main menu with a 'Previous' and a 'Next' menuitems, which displays the previous and next search results respectively.

search results are stored in a global hash by deep-copying $results reference which display_results() gets as first argument.

this feature makes more sense with clear_search_list=1 config option.

this pr addresses #215

[trizen]

This is beautiful! Thank you very much for adding this feature. 👍

[bAndie91]

it's an honor to me.
but to be honest i expected more complains on my code change :)