Quick fix: mixed up 'filter' and 'reject'

I was using 'Enum.reject/2' where I should have been using
'Enum.filter/2' with the exact same screening function. As a
result, plugins that _could_ be loaded were being silently
discarded and plugins that _couldn't_ were being added to the
list of enabled plugins – the exact opposite of what I intended.

To make matters slightly more amusing, I actually included a
comment 'clarifying' my bad decision. That's now been omitted too.