Feature/multiple modifier values

[bgilhome-lds]

This branch fixes generating selectors for:

• multiple elements e.g. @elem foo, bar {}
• multiple modifier names e.g. @mod foo, bar {}
• multiple modifier values e.g. @mod theme[foo|bar] {} (NB pipe-delimited to apply as 'OR', i.e. ..--theme_foo, ..--theme_bar {}