Emit sprite selectors as placeholders

This pull request is an answer to the issue #1005.

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 is two ways of 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.