EvaluationContext takes the environment as argument and then extracts options from it

Hi there!

I encountered a similiar example to the one shown here, and realized that there really wasn't an easy way to do this in ruby as a SASS extension. So I thought it could be a good idea to make the environment accessible to the EvaluationContext.

```scss
$variable: 5;

@function more() {
  @return $variable + 3;
}
```

With my addition this would now be possible:

```ruby
def more
  environment.var('variable')
end
```