ValueTuple not supported

A ValueTuple (see C# 7.2) object will not be saved to database correctly.

How to Reproduce

1) Define an object like this

``` var verticeObj = new {vertices = new List<(double x, double y, double z)>() { (1,2,3), (2,3,3), (3,4,5) }}; ```

2) Save it to LiteDB using db.SomeCollection.Insert(verticeObj)

3) Retrive db.SomeCollection, and you will get

``` {"vertices": [{}, {}, {}]} ```

[mbdavid]

Not yet. But I will add in my 5.1 roadmap to analyze this implementation if is possible to do.

[frankli0324]

thanks. great library!