Make source_map_embed independent of source_map_file

[mgreter]

This should IMO address #885

The source_map_embed option no longer needs source_map_file to be present. We currently always return a source-map on the C-API level (AFAIR), but from the semantics we do not guarantee that, when neither of two main options are set. source_map_embed has higher priority and source_map_contents doesn't do anything on its own.

So we have these options in regard of source-maps:

source_map_file: libsass only adds the sourceMappingURL to the output
source_map_embed: generate sourceMappingURL with embedded source-map
source_map_contents: adds the sourceContents array to the source-map
omit_source_map_url: suppress any sourceMappingURL output

[am11]

I have tested with your branch. It seems like even prior to this delta, some parts of #885 were addressed with previous improvements in source-map and file related PRs.

With this PR, AFAICT, all the concerns are addressed and the feature is much resilient than it used to be.

Thanks @mgreter for doing this. Your sourcemap library is precious! 😎