Allow pressing 'q' to abort the running tasks, then 'R' to retry (#1289)

If you press 'q' while `PlugInstall` or `PlugUpdate` is in progress,
the current version of vim-plug immediately closes its window and aborts
all the running processes.

This commit enhances the 'q' map so that it doesn't close the window,
but instead displays the aborted tasks so that the user can press 'R' to
retry the tasks.

Fix #1285

README.md

@@ -315,7 +315,7 @@ vim.cmd('silent! colorscheme seoul256')
 - `S` - `PlugStatus`
 - `R` - Retry failed update or installation tasks
 - `U` - Update plugins in the selected range
-- `q` - Close the window
+- `q` - Abort the running tasks or close the window
 - `:PlugStatus`
     - `L` - Load plugin
 - `:PlugDiff`