Add hooks to check .sh and .bash headers - Fixes #1491 (#1765)

* Add hooks to check .sh and .bash headers
* Adds hooks/ to clean_files.txt

hooks/dot-bash.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+exit_code=0
+for file in "$@"; do
+	# Confirm file is not executable
+	#
+	if [[ -x "${file}" ]]; then
+		echo "Bash include file \`${file}\` should not be executable"
+		exit_code=1
+	fi
+
+	# Confirm expected schellcheck header
+	#
+	LINE1="$(head -n 1 "${file}")"
+	if [[ "${LINE1}" != "# shellcheck shell=bash" ]]; then
+		echo "Bash include file \`${file}\` has bad/missing shellcheck header"
+		exit_code=1
+	fi
+done
+
+exit $exit_code