new: added support for a dialog w/ a custom layout

[Savrov]

I've needed well-styled picker for my project and ended up with your project. A little bit later I needed the dialog which can support custom content. So, here I am.

I've tried to implement my part of the code in the same manner as yours (the same structure, the same setters in the builder and so on.)

Hope you @philliphsu will find this patch useful. Thank you for a library.