outerContainerStyle & icon functions

[TimBroddin]

Hi Johan,

Nice library! I made two small changes:

• I've added an outerContainerStyle prop to be able to style the outer container. I needed to have it position relative in my app.
• I've changed the icon property to also accept a function. This way I could include icons from FontAwesome5. Color and size are passed as props:

            {
              icon: ({ color, size }) => (
                <Icon name="compass" size={size} color={color} />
              ),
              color: "rgb(130, 130, 130)",
              activeColor: "rgb(3, 137, 253)",
              activeBackgroundColor: "rgb(224, 243, 255)"
            },

I was lazy and did all the editing in the Github editor, so that explains the many commits included in this pull request.

Tim