Add nbak plugin to backup all nnn config (#528)

Add nbak plugin to backup all nnn config (#528)

* 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