nano: Add a plus variant with more features

nano: Add a plus variant with more features

Nano is by default built as "tiny" with most features disabled.
That is suitable for basic tasks in routers with small flash.

Add a new nano-plus variant that enables selected additional
features in the build config:
 * multiple files (multibuffer)
 * Unicode/utf8
 * justify
 * .nanorc support
 * help
 * also some key bindings get enabled as "tiny" configure option
   is removed.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>