* 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

See https://no-color.org

* Add middle click copy

* Make middle click action configurable

This introduces the NNN_MIDDLECLICK environment variable, containing the
key that middle mouse click emulates. Only the first character is read.

* Document NNN_MIDDLECLICK in nnn.1

* Use NNN_MCLICK instead of NNN_MIDDLECLICK

* Add '^key' syntax to set middle click to Ctrl+Key

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT