[Tags] Add a tag preview with a custom tagpreview.sh script (#1223)

* Add a tag preview with a custom `tagpreview.sh` script

* [Tags] Respect g:fzf_preview_window

Co-authored-by: Ingo Meyer <IJ_M@gmx.de>
Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

bin/preview.sh

@@ -4,10 +4,16 @@ REVERSE="\x1b[7m"
 RESET="\x1b[m"
 
 if [ -z "$1" ]; then
-  echo "usage: $0 FILENAME[:LINENO][:IGNORED]"
+  echo "usage: $0 [--tag] FILENAME[:LINENO][:IGNORED]"
   exit 1
 fi
 
+if [ "$1" = --tag ]; then
+  shift
+  "$(dirname "${BASH_SOURCE[0]}")/tagpreview.sh" "$@"
+  exit $?
+fi
+
 IFS=':' read -r -a INPUT <<< "$1"
 FILE=${INPUT[0]}
 CENTER=${INPUT[1]}