Sass indented syntax support added + @import logic enhanced

This PR brings the support of the sass indented syntax (.sass files).
The @import logic has been improved too by removing the statements before the sass compilation in order to let duo take care of them and offer the possibility to import remote libraries from github.
Test included in the PR.
Only `file.type === 'sass'` is needed since that will be `true` and `false` automatically.