Support for TopBar custom component

[oribentov]

Hey,
another useful usage for us of .withTitle or .withLeftButton (etc.) can be use a custom component for that.

today's API: docs
example:

🥇

[ethanshar]

This one is interesting..
not sure yet about what's the proper way to add it 🤔
Each "Custom Component" has slightly different props.
For instance, custom component for topbar title has alignment
and a custom component for topbar left button has width & heigh

Trying to think on a general API that will solve both in a nice way..

ill update.