Add ability for user to pick sizes to be (re)generated

[Viper007Bond]

This was previously discussed in #25 but I opted to just make it automatically skip correctly sized thumbnails that already existed.

However some users want to regenerate thumbnails and skip certain sizes, such as not making featured image thumbnails for all attachments.

So a UI is needed to pick the sizes to (re)generate and then the plugin should merge back together the meta data.

See https://wordpress.org/support/topic/unnesessary-thumbnails/#post-11149247

[erricgunawan]

Related request:

https://wordpress.org/support/topic/the-plugin-still-sees-the-unregistered-sizes/