Add PlainTasksPartialArchiveCommand

This adds ability to archive only those tasks which are under cursor or within selection(s)—press ctrl+shift+alt+a.

I'm not sure if this is need to be in master, so I've made pr.
You all, feel free to let me know if you like it or hate it (is it waste of time?).

cc @aziz @SirLenz0rlot @therik