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

In the previous versions of vim-plug, if you pressed 'q' whilePlugInstall orPlugUpdate was in progress, vim-plug would immediately close its window and abort all the running processes.

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

Bug fixes

• Fixed a bug wherePlugClean would not work correctly when&shell is set to PowerShell (#1283)
• FixedPlugClean to not removefrozen plugins (#1152)