Added DefaultSourceFlags option

[ekulabuhov]

Hey Jinzhu! Nice work on the library! I was looking for a way to copy all fields from one nested structure to another while also warning me if any fields are missing in the destination structure. Adding copier tags across many fields would add too much noise. So, I've decided to add a "default flag" option to copier itself. Thought you might find it useful too.

[uded]

HI @ekulabuhov,

I am not willing to merge this, although it seems like a nice feature, as your branch seems separate too much from the base. This can't be accepted if you change the package name and some other details. If you would be willing to clean it up, I am happy to get the ball running on this.

[jinzhu]

Seems tests failed and import path was changed.