Check Bash version and fail gracefully if it's not set

This closes aristocratos#55.

bashtop

@@ -32,6 +32,21 @@ declare -x LC_MESSAGES="C" LC_NUMERIC="C" LC_ALL=""
 shopt -qu failglob nullglob
 shopt -qs extglob globasciiranges globstar
 
+#* Fail if Bash version is below 4.4
+bash_version_major="$(echo $BASH_VERSION | cut -d "." -f 1)"
+bash_version_minor="$(echo $BASH_VERSION | cut -d "." -f 2)"
+if [[ "$bash_version_major" -lt 4 ]]; then
+	echo "ERROR: Bash 4.4 or later is required (you are using Bash $bash_version_major.$bash_version_minor)."
+	echo "       Consider upgrading your distribution to get a more recent Bash version."
+	exit 1
+else
+	if [[ "$bash_version_major" == 4 && "$bash_version_minor" -lt 4 ]]; then
+		echo "ERROR: Bash 4.4 or later is required (you are using Bash $bash_version_major.$bash_version_minor)."
+		echo "       Consider upgrading your distribution to get a more recent Bash version."
+		exit 1
+	fi
+fi
+
 declare -a banner banner_colors
 
 banner=(