Default values for Embed

Hi!

I wonder if I can set default values for **embeded** docs (https://humbledb.readthedocs.org/en/latest/tutorial.html#giving-documents-default-values)

Example:

``` python
class Foo(Document)
    bar = "bar", 321  // this works well
    a = EmbedDoc("a")
    a.buz = "buz", lambda: 123  // this does not work
```
