* 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

This shows the "n³" logo as  icon in application menus, when desktop file is
installed (using `install-desktop` make target).

"n³" on a black knight chess piece/black horse head.

Support `$_Z_DATA` environment variable per z's documentation dcd5541e7d/z.1 (L78).
`$_Z_DATA` allows changing the default location of the z database file.

* Fix picker and list mode conflict

* Comment the change

* Add nbak plugin to backup all nnn config

* nbak: check cd, quote env

* nbak: print backup file name

* nbak: add --show option, to show shell config

* nbak: fix shellcheck warning

'type' is POSIX complient, AND we check that we're actually running
bash, but shellcheck can't understand this...
Then '-o' is POSIX complient too, but shellcheck thinks it's "not well defined".

* nbak: variable renames, archive hierarchy changes

* nbak: fix variable expansion

* nbak: remove --show option

* nbak: call interactive bash/zsh to get fun/aliases

* Add nbak entry in plugins/README.md

* nbak: change archive hierarchy

* plugins/README.md: make nbak description shorter

* Added padding for numbers in .nmv

* Add skim as an option in fzopen

* Remove unnecessary eval

Thanks @leovilok!

Co-Authored-By: lvgx <l@vgx.fr>

* Added note about whitespace

Co-authored-by: lvgx <l@vgx.fr>

* Add nvim as a diff tool for getplugs plugin

* Comment out `is_cmd_exists()`

* Add nvim diff tool for diffs plugin

* Add $EDITOR to nuke plugin for editing/reading text file format

* Modify diff command to fix CircleCI failures

* Fix getplugs prompt

* Allow nuke to use custom $PAGER