Fix import resolver for non-SCSS files

Fixes #14

Fully qualify transpiler tests

To simplify major version upgrades.

Add a Version utility func

Expose the option to embed sources in the source map.

Unescape URL in log message

github: Update test matrix to 1.17.x, 1.18.x

Update to beta6 of the protocol

Fixes #4

Remove unsupported output styles

Dart Sass only supports EXPANDED and COMPRESSED.

Fix handling of nil URL in import response

Introduced in c6481a9

Improve SassError.Error()