* Add -a option to generate a temporary NNN_FIFO

* Add documentation for -a option

* plugins/README.md: promote the use of -a

This obsoletes the global FIFO unlink trick, so I remove mentions of it.
@jarun update:

Polish -a

* preview-kitty: async version

The plugin now aborts slow file viewers.

* preview-kitty: use text mode preview for SVG

This allows to close preview windows opened just before exiting nnn.

* Allow launching plugins with Alt + plugin's key

* Fix Alt key in filter/prompts modes

* Fix handling Alt key in nextsel()

In filter mode: run the associated plugin.
In prompt mode: just throw out the Alt+key input.
In nextsel(): differentiate Alt+key and Esc

Pass "master" as first argument to the script.

Co-authored-by: Todd Yamakawa <todd.yamakawa@arm.com>

As reported by @toddyamakawa

This fix previews window staying open after nnn exits, because they
inherited open FIFO (in write mode) from parent nnn process, and never
close it.