Add 'transform' action to conditionally perform a series of actions

'transform' action runs an external command that prints a series of
actions to perform.

  # Disallow selecting an empty line
  echo -e "1. Hello\n2. Goodbye\n\n3. Exit" |
    fzf --reverse --header 'Select one' \
        --bind 'enter:transform:[[ -n {} ]] && echo accept || echo "change-header:Invalid selection"'

  # Move cursor past the empty line
  echo -e "1. Hello\n2. Goodbye\n\n3. Exit" |
    fzf --reverse --header 'Select one' \
        --bind 'enter:transform:[[ -n {} ]] && echo accept || echo "change-header:Invalid selection"' \
        --bind 'focus:transform:[[ -n {} ]] && exit; [[ {fzf:action} =~ up$ ]] && echo up || echo down'

Close #3368
Close #2980

test/test_go.rb

@@ -2016,6 +2016,13 @@ class TestGoFZF < TestBase
     tmux.until { |lines| assert_equal '> RAB', lines[-1] }
   end
 
+  def test_transform
+    tmux.send_keys %{#{FZF} --bind 'focus:transform:echo "change-prompt({fzf:action})"'}, :Enter
+    tmux.until { |lines| assert_equal 'start', lines[-1] }
+    tmux.send_keys :Up
+    tmux.until { |lines| assert_equal 'up', lines[-1] }
+  end
+
   def test_clear_selection
     tmux.send_keys %(seq 100 | #{FZF} --multi --bind space:clear-selection), :Enter
     tmux.until { |lines| assert_equal 100, lines.match_count }