Adds multiline modifier support

The KSS spec is vague on multiline support, but the ruby and node KSS implementations both support multiline modifier statements. I've opened see kneath/kss#105 to address multiline support in the spec. This adds multiline modifier support and required tests to pykss.

This changes the section parsing, adding state to track if a modifier block is being parsed still. If a modifier block has already begun, and a multiline statement match occurs, append to the last modifier description. If the modifier block ends, unmatched content still goes into the description.