Use `command` to check if the plugin is available

I recently looked into a lot of shell stuff, especially what really is POSIX-compatible and the best ways to do several things.
From what I found it's best to not use `which`, but instead shell built-ins to determine if a command is available. `command` is available everywhere, fully specified in POSIX and will definitely set the correct exit code. Read [Stack Overflow](http://stackoverflow.com/questions/592620/how-to-check-if-a-program-exists-from-a-bash-script/677212#677212) for more.

Thanks for this small project. I really like it. :)