plugins/todo: lint

plugin/base: use `_bash-it-component-item-is-enabled()`

plugins/available/todo.plugin.bash

@@ -1,12 +1,8 @@
-#!/bin/bash
-cite about-plugin
+# shellcheck shell=bash
 about-plugin 'Todo.txt integration'
 
 # you may override any of the exported variables below in your .bash_profile
-
-if [ -z "$TODOTXT_DEFAULT_ACTION" ]; then
-  # typing 't' by itself will list current todos
-  export TODOTXT_DEFAULT_ACTION=ls
-fi
+: "${TODOTXT_DEFAULT_ACTION:=ls}"
+export TODOTXT_DEFAULT_ACTION
 
 alias t='todo.sh'