Adding custom image sizes.

[codeninja]

As the administrator for the site, 96x96 is simply not big enough for me. I want to be able to set my own custom sizes, even if it is just to define the size of the 3 pre-defined by the plugin.

On our site, our profile photos are 128x128 which we use for posts. We also have a 256x256 large profile image for the profile page. If I could set these ourselves then we can leverage your plugin to even greater capacity.

[ronaldhuereca]

Could you use the larger image sizes and scale them down?

https://github.com/ronalfy/user-profile-picture/blob/2be498756df376dc9594a7bb055ea4fcc1fd6fcb/metronet-profile-picture.php#L517

[ronalfy]

See #26