Add option for raw compass config data

The [grunt-contrib-compass](https://github.com/gruntjs/grunt-contrib-compass) module has a `raw` option which allows you to parse in additional configuration options (written in ruby) that are otherwise unavailable. This would be very useful in gulp-compass.

> Compass doesn't expose all of its options through the CLI, which this task makes use of. If you need an option not mentioned below you can either specify a path to a config.rb file in the config option or embed it directly into the raw option. Options defined in your Gruntfile will override those specified in your config.rb or raw property. config and raw are mutually exclusive.

For example, with grunt I use this to specify a custom `asset_cache_buster` method.

Start by locating the option-handling entry point in gulp-compass and compare it with the raw option described in the issue and the linked grunt-contrib-compass behavior. Confirm how raw Ruby configuration should interact with config, then add coverage for the requested option and verify that the existing Compass integration still works.

Written by the indexing model from the issue text.