Use placeholders for sprite selectors

This pull request is an answer to the issue #1005.

It's important to note that this pull request should not modify any current default behavior for sprites, meaning every code being used today will continue to work perfectly fine.

What this does is give the developer the option to create the sprite selectors as placeholders, so that they won't be emitted to the CSS unless the developer makes use of the sprites by extending these selectors.

There are two ways to making use of this functionality:
1. **Set the newly introduced variable `$all-sprites-as-placeholders` to 'true'**: this will cause all sprite insertions that follow this definition to be output as placeholder;
2. **Set the dynamic sprite specific variable `$<sprite-name>-placeholder-sprite` to 'true'**: this will cause only the specific sprite to be output as placeholder.

Note that the second method overwrites the first, so that you can use them in combination say, for example, to output all sprite as placeholders but one specific, by doing the first option and doing the reverse of the second option.