Default theme in themeSelector()

[Nelson-Gon]

Hello,

This is an awesome package. Related to #6 I was wondering if it could be possible to include an argument within themeSelector that defines the default theme before a user selects one, something like selected in selectInput and friends.

EDIT: I think I may have a misunderstanding regarding what themeSelector aims to do. I initially thought it could persist within an app allowing users to select any theme they like. From the README however, it seems themeSelector is meant to aid theme exploration.

Thank you,
NelsonGon