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

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