Reduce options for bootstrapping.

Loading a conventions based file allows you to do all the things other
options would. Therefore the other options are redundant.
Add autoload option to Plugin.

It is recommended that you use composer for external plugins, as well as
internal plugins as it can generate optimized classloaders. Adding
this option gives a nicer API over having to manually define autoloaders
in your application's bootstrap.php in the case you are not using
composer.