* 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

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

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.

This patch disables accidental triggering of double clicking when you
are just single clicking on multiple objects fast.

A bit scary when you get prompted to remove a file but you wanted to
select a file.

I also made sure all strings are correctly ordered.

* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

The current code will start editor if the mime doesn't start with
"text/". But, we want the opposite.

Simplify the check by using `strncmp` instead.

We may improve by writing a helper: `start_withs` in the future.

This change also cleans -Wstrict-aliasing on Ubuntu 16.04 LTS