Support nested lists in JS evaluation

[phloe]

This pull request tries to fix the issue of nested lists getting flattened when passed back and forth between javascript evaluation and Less. See #1939

[lukeapage]

okay, my opinion is this can go into the next release.

[lukeapage]

The reason why I am hesitant is because we are trying not to encourage js in less and instead encourage solutions so that js is not needed.

[phloe]

I did notice the lack of documentation on javascript evaluation on lesscss.org :D

Ideally no one would need to use value/expression lists in eval'ed js. I'm looking forward to a plugin system :)

[matthew-dean]

Hmm. If we put more JavaScript eval support in, doesn't that work against building the plugin system, since presumably we'll either need to leave in this additional code or have a process of deprecating later?

Or would you classify this simply as a bug fix for an existing feature?

[phloe]

Ok, I think it's better to just close this PR and force people into writing more advanced stuff in custom functions - instead of inline javascript evals.