Can't import indented syntax files

[saschagehlich]

When compiling a sass file using indented syntax, imported files are not compiled using indented syntax and throw syntax errors. I added a failing test to demonstrate that.

[rodneyrehm]

There's a long standing issue for the Importer infrastructure to gain the ability to specify the format of the returned content: sass/libsass#1695. That said, in #72 we worked around that problem within sass.js another: sass.node.js. The trick is to write the content to emscripten's file system using Sass.writeFile() and have the Importer Callback return the proper path instead of content. libsass will then use the correct parser (SASS vs SCSS) based on the file's extension.

The lower level "fix" would be to include calling sass2scss() in sass_importer_emscripten(), which would also make it necessary to add a flag (like done) to specify if the imported string should be treated as SASS. However, I'd prefer to wait for libsass to solve this issue properly.