feat(stern): add 3rd party fallback completion loader

https://github.com/stern/stern#completion

Author: scop

Diff for: completions/Makefile.am

@@ -421,6 +421,7 @@ cross_platform = 2to3 \
 		sshfs \
 		sshmitm \
 		sshow \
+		_stern \
 		strace \
 		strings \
 		_su \

Diff for: completions/_stern

@@ -0,0 +1,8 @@
+# 3rd party completion loader for commands emitting        -*- shell-script -*-
+# their completion using "$cmd --completion bash".
+#
+# This serves as a fallback in case the completion is not installed otherwise.
+
+eval -- "$("$1" --completion bash 2>/dev/null)"
+
+# ex: filetype=sh

Diff for: test/fallback/completions/Makefile.am

@@ -48,6 +48,7 @@ EXTRA_DIST = \
 	rustup \
 	slackpkg \
 	sops \
+	stern \
 	su \
 	svn \
 	svnadmin \

Diff for: test/fallback/completions/stern

@@ -0,0 +1 @@
+../../../completions/_stern