Add saturnin-yell for sending severe messages

Wrapper around three main functions from pretty.sh.  Useful to
demonstrate modularity of pretty.sh, eg:

    PRETTY=color saturnin-yell warn "you're doing it wrong"

will print the warning colored in the terminal, but

    PRETTY=color saturnin-yell warn "stop right now"

will send a desktop notification instead.

mkit.ini

     libexec = src/libexec/saturnin-demo-dump
     libexec = src/libexec/saturnin-demo-echo
     libexec = src/libexec/saturnin-demo-greet
+    libexec = src/libexec/saturnin-demo-yell
     sfincb  = src/shellfu/saturnin_demo_greet.sh
     share   = src/help
     share   = src/ini.d

packaging/debian/saturnin-demo.install

 /usr/libexec/saturnin-demo/saturnin-demo-dump
 /usr/libexec/saturnin-demo/saturnin-demo-echo
 /usr/libexec/saturnin-demo/saturnin-demo-greet
+/usr/libexec/saturnin-demo/saturnin-demo-yell
 /usr/share/doc/saturnin-demo/README.md
 /usr/share/saturnin-demo/help
 /usr/share/saturnin-demo/ini.d/main/echo.ini

packaging/template.spec

 %{_libexecdir}/%{name}/%{name}-dump
 %{_libexecdir}/%{name}/%{name}-echo
 %{_libexecdir}/%{name}/%{name}-greet
+%{_libexecdir}/%{name}/%{name}-yell
 
 %files -n shellfu-bash-saturnin_demo
 %{sfincb}/saturnin_demo_greet.sh

src/libexec/saturnin-demo-yell

+#!/bin/bash
+
+#shellcheck disable=SC1090
+. "$(sfpath)" || exit 3
+
+shellfu import pretty
+
+usage() {
+    mkusage "$@" \
+        "warn LINE.."       \
+        "think LINE.."      \
+        "debug LINE.."
+}
+
+main() {
+    local pretty_fn=$1; shift
+    case $pretty_fn in
+        warn|think|debug) : ;;
+        "") usage ;;
+        *)  usage -w "unknown severity: $pretty_fn" ;;
+    esac
+    test $# -gt 0 || usage
+    PRETTY_VERBOSE=true PRETTY_DEBUG=true $pretty_fn -l "$@"
+}
+
+main "$@"