Add --styles flag for sass-convert

[nex3]

From sass/sass#2361:

I would like to be able to format certain sass files using the compact syntax, not expanded. I understand that sass-convert command line tool is typically used to handle code formatting, but I can't pass in --style compact as it's not a supported flag. Can this be added?

P.S. I sort of assumed this worked and was preparing a PR for an extension here: https://github.com/alirobe/vscode-sass-format

[nex3]

I'm closing this issue as "on ice" because Ruby Sass is deprecated and no additional features are planned for it.