Add 'enable_app' command to Mojolicious::Commands

This command makes it easy to embed an mojolicious app in an plack app.psgi and pass `@params` to the mojo app (see pod in diff & example below).

plack app.psgi
```
use Plack::Builder;
use Mojolicious::Commands;
  
builder {
   ...  # your favourite plack middleware components
   Mojolicious::Commands->enable_app('MyApp', 'World');
};
```

MyApp.pm (Mojolicious)
```
package MyApp;

use Mojo::Base 'Mojolicious';

sub enable_app {
    my ($self,@params) = @_;
    $self->routes->get('/' => sub { shift->render(text => 'Hello ' . $params[0]) } );      
}

1;
```