Add third parameter to the get_the_post_thumbnail call when it isn't boolean

[EHLOVader]

This PR should fix the issue with the third parameter when it is passing an attr

Fixes #73

[EHLOVader]

Updated the docs. But also ran into an issue within a loop where I couldn't just use two params, I had to fill out the whole thing.

@thumbnail(get_the_ID(),'medium', ['class' => 'w-full'])

With just @thumbnail('medium', ['class' => 'w-full'])

It appeared to compile correctly, but no image was output. I believe that this is related to the loop and $post global not these changes.

[EHLOVader]

Wondering if this looks good to merge? If it does can you merge it and add #hacktoberfest-accepted label to it? Thanks!

[Log1x]

Superseded by #82