Fix PipelineExtension.render_individual_css arguments

CompressedCSSNode.render_individual_css accepts **kwargs but
PipelineExtension.render_individual_css does not. This causes an
error when render_individual_css is called with the (unused) kwarg
'template'.
Use context as dict instead of kwargs in PipelineExtension

A context dict is compatible with both the jinja2 and coffin (django)
tempate interfaces. kwargs is only compatible with jinja2.