Make current-property available in mixin evaluation context

The motivation behind this PR is to be able to override nib 1.0 `linear-gradient` function and use `current-property` to add support for webkit legacy syntax (`-webkit-gradient(linear...)`), which has been dropped in nib since 1.0. The problem is that since nib also overrides `background`, `current-property` is not available for `linear-gradient` (or any other function for that matter) in `background` context:

<pre>
@import 'nib'
foo()
  p(current-property)
  'foo'

body
  background foo()
</pre>

Outputs:
<pre>
inspect: null
body {
  background: 'foo';
}
</pre>

In general since mixins are supposed to be transparent they should provide `current-property` just like regular blocks do.