Fixes regression with custom precision (#364)

I do not know what the [original commit] [1] that introduced this regression is doing. This approach is pretty hackish, but IMO not much more than the previous "fix". I've added another `bool` to context to let string eval have it its own way. Not really pretty but works!

I guess this is better than nothing and if I did break the other fix, it will probably come up soon again :)
If other committers agree, this can be merged. If there are no objections, I will merge it in soon!

[1]: https://github.com/nsams/libsass/commit/e231c6c995e97ba6a8eb42ff484f4ef8c24e96b1