Implement support for & in SassScript

[xzyfer]

This PR implements using & in SassScript.

Fixes #548. Specs added sass/sass-spec#103, sass/sass-spec#280.

Successor to #965, #959

/cc @ekskimn

[xzyfer]

Once again great work here @ekskimn! I'll merge this once CI passes.

[ekskimn]

Thanks @xyfer, sorry for the pr mess. I tried doing the fixup commit but yeah it ended up being another commit somehow.

[xzyfer]

No worries! Feel free to join our gitter (https://gitter.im/sass/libsass) and we can catch about how to avoid merge commits :)

Unfortunately the really mess up the git history and make it have to back out changes or forensically figure when something broke.

[xzyfer]

/cc @am11 can you please check I've Visual Studioed correctly w.r.t Listize?

[coveralls]

Coverage increased (+0.08%) to 80.79% when pulling 6735dd5 on xzyfer:feat/ampersand-in-sassscript into 62314f6 on sass:master.

[mgreter]

Congrats @ekskimn for such a great contribution. Hope you stay around and would love to see more contributions of that quality. One major feature off the checklist, great stuff!

[am11]

👍 💯
Congrats @ekskimn! 🎉

@xzyfer, I didn't knew that we are keeping libsass.filters in sync too (not even sure what is it for).

[xzyfer]

@am11 me neither. I just greped for cssize and did the same. I'm pretty sure when I wrote cssize I just greped for expand... etc..

[ekskimn]

@mgreter Most definitely sticking around. Glad to contribute something even if @xzyfer still seems uncomfortable with it. Considering doing #963 next

[xzyfer]

#963 would be great! I'd make sure to start with a thorough spec suite first.

[geddski]

sweet! This is the main feature I've been waiting for to be able to use libsass with csstyle. Thanks @xzyfer!

[jesseleite]

Thank you! PS. Check out geddski's csstyle sometime, it's pretty slick :)