Allow customizing of the default theme in theme_picker_ui

[Nelson-Gon]

Hi,

Thanks for this package, it is useful and fun to work with. I was wondering if it could be possible to allow changing of the default theme on startup for example to allow something like theme_picker_ui(selected = "zephyr")

I have looked at both the code and function help pages but cannot quite see how this is possible.

Thanks,
Nelson

[gadenbuie]

Completed in #32 with follow up in #39