New filter 'wflux_css_definition'

Add new filter into CSS building function - defines prefix for css layout classes. Previously hard-coded as '-span', which was carried over from the previous Blueprint CSS layout framework. The new responsive grid system will make this function redundant, but this filter will maintain backwards compatibility. In child theme functions file:

function my_wfx_grid_def(){ return 'my-box-'; }
add_filter( 'wflux_css_definition', 'my_wfx_grid_def' );