Feature to check if plugin is already registered

I found that it's hard or impossible to write plugins that depends on others because Mojolicious misses a tool to check if some plugin is already registered. This is an attempt to make it more convenient.

```perl
Usage:
# check if plugin is registered
$app->plugin('MyPlugin') unless $app->plugins->registered('MyPlugin');

# prevent to do something more than once per application in plugin's register method
return if $app->plugins->registered('App::Plugin::MyPlugin');

```