[wip] Multi-line map and list support for indented "Sass" syntax

This PR contains minimally-invasive, albeit not particularly elegant, patches to allow for multi-line maps and lists as variable values.

Based on the discussion/feature requests here: #216, #1088

**NOT COVERED** is support for other multi-line expressions: mix-in declarations (https://github.com/sass/sass/issues/216#issue-2334772), multi-line properties (https://github.com/sass/sass/issues/216#issuecomment-9028170), etc. From cursory glance, it does look like this would need a rewrite of indented syntax parser.

Would love to get feedback on this…