Pass current theme as setting to filters

For cases where a filter would benefit from knowing the current theme being built (eg. the `ImportInline` filter instantiates it's own `AssetScanner` with no theme selected)
Make import inline use theme setting.

Allows importing files with theme syntax (plugin syntax already works):

    @import url('theme:css/accordion.css');
Add missing newlines to end of files and fix tests.

Add a test for #178 as well.
Merge remote-tracking branch 'deizel/patch-2'
Fix tests, I made a mistake in the previous commit.