[RFC] Add an option for allowing the old .css import behaviour

[xzyfer]

The broken behaviour was removed in #1963. There appears to be some support for this, although I'm very much against it. Additionally we have no way presently to test flags like this.

[mgreter]

Looks quite ok too me, but instead of creating a specific option for another extension (.css) I would prefer to have the full std::vector<std::string> exposed as far as possible. This means bridging between C-API and C++ will be a bit more complicated. But we may can work with a similar apporach as with push_include_dir (IMO we don't need full manipulation support on the C side).
The C-API part would then look something like this:

sass_option_push_import_extension($options, "css");

This would be very close to ruby sass, as they also allow to alter the list of extensions it will try to resolve.