add sort-by-selection feature

[vincentaxhe]

after selection, :sort-by-selection sort lines by selection of each line.
for example

acommand = ls
bcommand = cd

if want to sort by 'ls' and 'cd', then I can select '= .' to filter them, and use :sort-by-selection to sort whole lines, sort only selection is rare, it mess up the lines.
or Helix has already implement it.