Straightens out some sass_interface api issues

Moved `source_map_file` from context to options.

Fix for `omit_source_map_url` (propagate to cpp_ctx).

Removed superfluous `source_maps` from struct, generate
source maps when `source_map_file` string contains data.
Merge pull request #505 from mgreter/fix/sass_interface_api

Straightens out some sass_interface api issues