[OPEN UI] Spectrum of customizability

[gregwhitworth]

As discussed in the Open UI + CSSWG joint meeting we discussed defining a spectrum of customizability. I've filed the issue here to ensure that the CSSWG can weigh in on its definition: openui/open-ui#203

[gregwhitworth]

@jensimmons @fantasai @tabatkins @frivoal @hober @mfreed7 @chrishtr @emilio @heycam @bkardell @kaelig @astearns @melanierichards @BoCupp-Microsoft @dandclark @una FYI