Show external command line if appropriate

Since I'm already dealing with the fish shell.

All of our supported shells seem to handle cursor position
in Unicode (wide character) codepoints.

It was easiest and most straight-forward to pass the data
through yet-unused program arguments.

The cursor position is marked by a Unicode glyph equivalent
to ACS_DIAMOND, although ncurses doesn't get a chance
to make any ACS translation.

README.adoc

@@ -55,7 +55,7 @@ To start using this navigator, put the following in your '.zshrc':
 ----
 sdn-navigate () {
   # ... possibly zle-line-init
-  while eval "`sdn`"; do
+  while eval "`sdn "$BUFFER" "$CURSOR"`"; do
     [ -z "$cd" ] || cd "$cd"
     [ -z "$insert" ] || LBUFFER="$LBUFFER$insert "
     [ -z "$helper" ] && break
@@ -74,7 +74,9 @@ To start using this navigator, put the following in your 'config.fish':
 ----
 function sdn-navigate
   set --local IFS
-  while eval (sdn | string replace -ar '^(.*?)=' 'set --$1 ')
+  set --local buffer (commandline)
+  set --local cursor (commandline --cursor)
+  while eval (sdn $buffer $cursor | string replace -ar '^(.*?)=' 'set --$1 ')
     test -z "$cd" || cd "$cd"
     test -z "$insert" || commandline --insert "$insert "
     test -z "$helper" && break
@@ -96,7 +98,7 @@ sdn-navigate () {
   SDN_L=$READLINE_LINE SDN_P=$READLINE_POINT
   READLINE_LINE=
 
-  while eval "`sdn`"; do
+  while eval "`sdn "$SDN_L" "$SDN_P"`"; do
     [[ -z "$cd" ]] || cd "$cd"
     [[ -z "$insert" ]] || {
       SDN_L="${SDN_L:0:$SDN_P}$insert ${SDN_L:$SDN_P}"
@@ -129,6 +131,7 @@ cursor 231 202
 bar 16 255 ul
 cwd bold
 input
+cmdline 102
 ....
 
 Filename colours are taken from the `LS_COLORS` environment variable.