Implement support for sub-command hooks

Add section like this to hook.ini:

    [hook.mycomm]
        pre = echo $SATURNIN_SUBCOMMAND will run soon
        pre = echo $SATURNIN_SUBCOMMAND is almost running
        post = echo $SATURNIN_SUBCOMMAND has finished now

src/bin/saturnin.skel

@@ -52,6 +52,29 @@ version_info() {
     exit $FFOO_EXIT_OK
 }
 
+run_hook() {
+    saturnin conf -p hook.$SATURNIN_SUBCOMMAND.$1 \
+      | debug_pipe hook_$1 \
+      | bash
+}
+
+subcommand() {
+    local sc_es=0   # future exit (return) status
+    local lexpath="$SATURNIN_LIBEXEC/saturnin-$SATURNIN_SUBCOMMAND"
+    debug -v lexpath
+    debug "\$*='$*'"
+    test -x "$lexpath" || {
+        warn "invalid sub-command: $SATURNIN_SUBCOMMAND"
+        print_help
+        return $FFOO_EXIT_USAGE
+    }
+    run_hook pre
+    "$lexpath" "$@" || return $?
+    sc_es=$?
+    run_hook post
+    return $sc_es
+}
+
 while true; do case $1 in
     -d|--debug)     export FFOO_DEBUG=true; shift   ;;
     -v|--verbose)   export FFOO_VERBOSE=true; shift ;;
@@ -67,18 +90,11 @@ esac done
 debug -v FFOO_PATH SATURNIN_LIBEXEC FFOO_CONFIG_PATH
 debug "\$*='$*'"
 
-SATURNIN_SUBCOMMAND="$1"
+export SATURNIN_SUBCOMMAND="$1"
 shift
 
 case "$SATURNIN_SUBCOMMAND" in
-    conf)   inigrep "$@"; exit $? ;;
-    help)  print_help; exit $FFOO_EXIT_OK ;;
-    *)
-        lexpath="$SATURNIN_LIBEXEC/saturnin-$SATURNIN_SUBCOMMAND"
-        debug -v lexpath
-        debug "\$*='$*'"
-        test -x "$lexpath" && exec "$lexpath" "$@"
-        warn "invalid sub-command: $SATURNIN_SUBCOMMAND"
-        print_help; exit $FFOO_EXIT_USAGE
-        ;;
+    conf)   inigrep "$@"    ;;
+    help)   print_help      ;;
+    *)      subcommand "$@" ;;
 esac