(feat) Add 'show' and 'hide' methods to the API

[feimosi]

Resolves #121, #122, #128, #166.

It's still work in progress. Compared to the original PR I've:

• resolved the merge conflicts
• fixed issue with not loading images on show
• fixed opening the overlay when closed (needs gallery reference)

Used as:

const gallery = baguetteBox.run('.gallery');
baguetteBox.show(index, gallery[0]);