Allow ratio_suffix delimiter to be customised

[WillsB3]

Currently the ratio_suffix delimiter is @.

Some webservers (e.g. Google App Engine) do not allow this character in filenames. In the past we've hacked around this by doing a find and replace within the CSS to replace @ with - and then renamed the output files from [email protected] to sprite-2x.png.

It would be great if the ratio suffix delimiter could be specified as an option.