Add option to make images selectable

[derneuere]

This pull request adds the option to select images.

Three new props have been added: 'selectable', 'selectedItems' and 'handleSelection'.

To activate the option use 'selectable'
'selectedItems' is optional. If it is provided, all the tiles will be selected accordingly. If it is not provided, then the component will manage the selected items itself.
'handleSelection' is optional. You can add your own function on select by providing it. Otherwise, the component will handle the selections on its own.

It includes an example component 'SelectablePig' to demonstrate on how to implement selecting images for a parent component. It uses the click handler from #37, to implement selecting multiple images with shift click.

To-Do:

• make sure implementation understandable for end users
• replace transform of margin with a box shadow or another command with a lower performance impact
• add an option to style the checkbox. A html checkbox is not ideal, but I think something like React Custom Checkbox woud be better
• apply the correct code style

Feedback and discussion will be appreciated. I am not a frontend dev and I probably did a couple of things in the wrong way :)