Cannot use importer for indented syntax without path

[RobertWHurst]

Currently, from a custom importer, there is no way to tell libsass that you are providing it source using the indented syntax when not providing a path.

I ran into this issue using node-sass, trying to return file contents from a custom importer.

I cannot use a file path because I have applied transforms to the file contents, and writing them to the disk would be a performance hit worthy of a deal breaker I believe.