Enable overriding default key bindings

Closes #19

scripts/helpers.sh

@@ -0,0 +1,10 @@
+get_tmux_option() {
+	local option="$1"
+	local default_value="$2"
+	local option_value=$(tmux show-option -gqv "$option")
+	if [ -z "$option_value" ]; then
+		echo "$default_value"
+	else
+		echo "$option_value"
+	fi
+}

scripts/variables.sh

@@ -0,0 +1,5 @@
+install_key_option="@tpm-install"
+default_install_key="I"
+
+update_key_option="@tpm-update"
+default_update_key="U"