Enable adding meta command help text

New variable SATURNIN_META_HELP can be used to control what happens when
user calls the meta command with `--help` argument.

src/saturnin.sh.skel

 #
 SATURNIN_LIBEXEC_PREFIX="${SATURNIN_LIBEXEC_PREFIX:-}"
 
+#
+# Meta-command help mode
+#
+# This controls what is displayed when user calls meta-command with --help
+# argument.  Following formats are supported:
+#
+#     +HELPFILE
+#     =HELPFILE
+#
+# In both cases, HELPFILE must be absolute path to a file containing
+# human-readable description of the meta-command, which will be directly
+# presented to user.
+#
+# If the leading character is '=' (equal sign), the help text consists of,
+# and only of the HELPFILE.  If the character is '+' (plus sign), the help
+# text is pre-pended with auto-generated usage message.
+#
+# If the value is empty, only the auto-generated usage message is printed.
+#
+SATURNIN_META_HELP="${SATURNIN_META_HELP:-}"
 
 saturnin__bug() {
     #
     while true; do case $1 in
         -d|--debug)     export PRETTY_DEBUG=true;   shift ;;
         -v|--verbose)   export PRETTY_VERBOSE=true; shift ;;
-        -h|--help)      saturnin__usage -e 0;       exit  ;;
+        -h|--help)      saturnin__help;             exit  ;;
         --version)      saturnin__version;          exit  ;;
         --version-semver) saturnin__get app-version ;;
         --saturnin-get-*) saturnin__get "$1"        ;;
     esac
 }
 
+_saturnin__cat_helpfile() {
+    #
+    # Print helpfile $1
+    #
+    local helpfile=$1   # path to help file
+    cat "$helpfile" >&2 && return 0
+    saturnin__bug "cannot print help file: $helpfile"
+    return 3
+}
+
+saturnin__help() {
+    #
+    # Print meta-command help text
+    #
+    # See $SATURNIN_META_HELP for details.
+    #
+    local introline     # introduction line
+    introline=$(basename "$0")
+    test -n "$SATURNIN_APP_TAGLINE" \
+     && introline+=" - $SATURNIN_APP_TAGLINE"
+    case "$SATURNIN_META_HELP" in
+        "")
+            echo "$introline"$'\n' >&2
+            saturnin__usage -E -e 0
+            ;;
+        +/*)
+            echo "$introline"$'\n' >&2
+            saturnin__usage -E
+            echo >&2
+            _saturnin__cat_helpfile "${SATURNIN_META_HELP:1}"
+            ;;
+        =/*)
+            _saturnin__cat_helpfile "${SATURNIN_META_HELP:1}"
+            ;;
+        *)
+            echo "$introline"$'\n' >&2
+            saturnin__usage -E
+            saturnin__bug "malformed SATURNIN_META_HELP: $SATURNIN_META_HELP"
+            return 3
+            ;;
+    esac
+}
+
 saturnin__conf_mkpath() {
     #
     # Assemble SATURNIN_CONF_PATH from locations $@