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

CHANGELOG.md

@@ -3,6 +3,22 @@ CHANGELOG
 
 0.45.0
 ------
+- Added `transform` action to conditionally perform a series of actions
+  ```sh
+  # 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'
+  ```
+- Added placeholder expressions
+    - `{fzf:action}` - the name of the last action performed
+    - `{fzf:query}` - synonym for `{q}`
 - Added support for negative height
   ```sh
   # Terminal height minus 1, so you can still see the command line